New planner task detail shows first day of week sunday instead of monday

Question

New planner task detail shows first day of week sunday instead of monday

CemOsmanKAMOY-6857 20

Hi,

I have difficulties to set the first day of week to Monday inside the Planner in the task detail popup.

All regional settings in my local pc, microsoft enviroment, teams, sharepoint settings are set to Turkish regional date&time.

All over the apps Monday is the first day. Even in planner when I click on the date of any task the date picker opens with the correct settings. Only when I open task details, in there the date picker shows a different structure with Sunday as first day. See two pictures below. What is wrong and how can I fix that?
I've tried the within teams, different browsers and in incognito mode as well.
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Answer accepted by question author

Liora D 16,225 Microsoft External Staff Moderator

Dear @CemOsmanKAMOY-6857,

I hope you’re having a good day.

Thank you for your detailed description and especially for the clear screenshots, they are very helpful in understanding what you are seeing.

From your images, I can see that your regional configuration is generally working correctly. In the main Planner view, the calendar follows the Turkish format with Monday as the first day of the week. However, in the task details popup, the calendar switches to Sunday as the first day, which creates the inconsistency you described.

To make sure we rule out all possible configuration influences, I would recommend checking your browser language/region settings, as some web components can still reference these values when rendering parts of the UI.

For example, in Microsoft Edge you can check this as follows:

Open Settings (You can also open: edge://settings/languages)

Go to Languages > Ensure that Turkish is added and set as the preferred display language

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After adjusting, restart the browser and test again

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You may also want to quickly verify the effect by opening a new tab or another Microsoft service (such as Outlook on the web) to see whether date formats and language are consistently displayed in Turkish.

From my own observations when testing in a similar setup, when the regional settings are correctly configured in Microsoft 365 (including Teams and SharePoint), Planner generally displays the calendar correctly, as you are already seeing in your first screenshot. However, some parts of the Planner interface may rely on different UI components. These components can sometimes be influenced by browser language settings, fallback defaults (such as en‑US), or differences in how localization is applied across the interface.

Because of this, it is possible to see mixed behavior within the same Planner experience, where one date picker follows your region settings and another does not, even though your overall configuration is correct.

I hope this information is helpful. If you still encounter the same behavior after checking the browser settings, please let me know, I would be very happy to take a closer look with you.

Thank you again and have a great day!

Warm regards,

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CemOsmanKAMOY-6857 20 Reputation points

2026-05-23T16:48:23.9733333+00:00

Hi Liora,

thanks for the quick response. Good point.
But the browser has the same settings. I'm mainly using edge but I already made several tests with incogni mode in edge and I also installed chrome for this purpose and tested there too. Same result.
Liora D 16,225 Reputation points Microsoft External Staff Moderator

2026-05-25T15:30:08.9533333+00:00
Hi Cem,

Thank you again for taking the time to test this thoroughly and for sharing the screenshots.

I’ve tried to reproduce your exact scenario on my side with a similar setup (regional settings, Turkish language, Planner usage in browser and Teams). In my environment, the behavior updates correctly once the browser or profile language is aligned. I also checked on another machine to be sure, and I was not able to reproduce the same issue you’re seeing.

Based on what you’ve described, you’ve already adjusted all the relevant settings (Microsoft 365 profile, browser language, system locale, and tested across browsers and clean sessions). That means the usual configuration-related causes are effectively ruled out.

To further isolate the behavior, it would be helpful if you could try one additional verification:

Test the same scenario on a different computer (if possible), using your account

If available, try with another user account in the same environment (or a colleague’s account)

You can also try this via your Microsoft account pagto confirm how your profile language is applied across services

This helps determine whether the behavior is tied specifically to your user profile or to your tenant/environment.

I truly appreciate the effort you’ve put into testing this so carefully and keeping me updated.

Please feel free to keep me posted if you notice any change or find anything new. I’m always here and happy to continue working through this with you.
Liora D 16,225 Reputation points Microsoft External Staff Moderator

2026-05-26T14:52:19.2833333+00:00

Hi Cem,

I just wanted to follow up to see how things are going with the information I shared above.

If you’ve had a chance to review it and have any updates, questions, or if you ran into any difficulties along the way, please don’t hesitate to let me know. I’m more than happy to assist further if needed.

It would be great to hear back from you!

Warm regards,

Liora.
CemOsmanKAMOY-6857 20 Reputation points

2026-06-01T06:07:17.6366667+00:00

Hi Liora,

We had national holdiays last week.

Today I started using it and surprise, it's fixed. I guess Microsoft engineers fixed it meanwhile. I did not made any changes since then. 😂
Liora D 16,225 Reputation points Microsoft External Staff Moderator

2026-06-01T15:09:49.1966667+00:00

Hi Cem,

That’s honestly the best kind of update to see, thank you for coming back and sharing it.

I’m really glad to hear that the behavior has now corrected itself on your side.

Thank you again for your patience and for taking the time to test things so thoroughly, it really helped narrow things down. If you notice anything similar in the future or run into other quirks, feel free to reach out anytime!

If you feel that the discussion here helped you get clarity or direction while troubleshooting, you might also consider marking it as the “Accepted answer.” Doing so will pin the thread, making it easier for others facing a similar situation to discover your question and benefit from the insights you’ve shared, especially when they’re not quite sure where to start.

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CemOsmanKAMOY-6857 20 Reputation points

2026-05-23T16:48:23.9733333+00:00

Hi Liora,

thanks for the quick response. Good point.
But the browser has the same settings. I'm mainly using edge but I already made several tests with incogni mode in edge and I also installed chrome for this purpose and tested there too. Same result.
Liora D 16,225 Reputation points Microsoft External Staff Moderator

2026-05-25T15:30:08.9533333+00:00

Hi Cem,

Thank you again for taking the time to test this thoroughly and for sharing the screenshots.

I’ve tried to reproduce your exact scenario on my side with a similar setup (regional settings, Turkish language, Planner usage in browser and Teams). In my environment, the behavior updates correctly once the browser or profile language is aligned. I also checked on another machine to be sure, and I was not able to reproduce the same issue you’re seeing.

Based on what you’ve described, you’ve already adjusted all the relevant settings (Microsoft 365 profile, browser language, system locale, and tested across browsers and clean sessions). That means the usual configuration-related causes are effectively ruled out.

To further isolate the behavior, it would be helpful if you could try one additional verification:

Test the same scenario on a different computer (if possible), using your account

If available, try with another user account in the same environment (or a colleague’s account)

You can also try this via your Microsoft account pagto confirm how your profile language is applied across services

This helps determine whether the behavior is tied specifically to your user profile or to your tenant/environment.

I truly appreciate the effort you’ve put into testing this so carefully and keeping me updated.

Please feel free to keep me posted if you notice any change or find anything new. I’m always here and happy to continue working through this with you.
Liora D 16,225 Reputation points Microsoft External Staff Moderator

2026-05-26T14:52:19.2833333+00:00

Hi Cem,

I just wanted to follow up to see how things are going with the information I shared above.

If you’ve had a chance to review it and have any updates, questions, or if you ran into any difficulties along the way, please don’t hesitate to let me know. I’m more than happy to assist further if needed.

It would be great to hear back from you!

Warm regards,

Liora.
CemOsmanKAMOY-6857 20 Reputation points

2026-06-01T06:07:17.6366667+00:00

Hi Liora,

We had national holdiays last week.

Today I started using it and surprise, it's fixed. I guess Microsoft engineers fixed it meanwhile. I did not made any changes since then. 😂
Liora D 16,225 Reputation points Microsoft External Staff Moderator

2026-06-01T15:09:49.1966667+00:00

Hi Cem,

That’s honestly the best kind of update to see, thank you for coming back and sharing it.

I’m really glad to hear that the behavior has now corrected itself on your side.

Thank you again for your patience and for taking the time to test things so thoroughly, it really helped narrow things down. If you notice anything similar in the future or run into other quirks, feel free to reach out anytime!

If you feel that the discussion here helped you get clarity or direction while troubleshooting, you might also consider marking it as the “Accepted answer.” Doing so will pin the thread, making it easier for others facing a similar situation to discover your question and benefit from the insights you’ve shared, especially when they’re not quite sure where to start.

Answer 1

This behavior is caused by how different parts of the Microsoft 365 ecosystem and apps consume regional and “first day of week” settings, and not by a misconfiguration in the environment.

From the available information, Planner and related task experiences can use different date‑picker components internally. Some experiences (for example, the board view date picker) correctly honor the tenant or profile regional settings, while others (such as the task‑details pane) can fall back to a default where Sunday is treated as the first day of the week. This is similar to other Microsoft 365 experiences where newer or separate apps don’t fully respect the Outlook/Windows regional settings yet—for example, Microsoft To Do versus legacy Outlook Tasks.

Because of this, there is currently no setting in Planner, Teams, or Windows that will force the task‑details date picker to start on Monday if that specific control is using its own default. The regional settings already configured (Windows, Outlook, SharePoint, Microsoft 365 profile) are correct and are why other date pickers show Monday as the first day of the week.

The only practical options at this time are:

Continue using the views where the date picker already honors Monday as the first day of the week.
Monitor Planner and Teams updates, as the behavior depends on how those clients implement regional settings support.

There is no additional configuration or fix available in current documentation to change the first day of week specifically for the Planner task‑details date picker.

References:

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New planner task detail shows first day of week sunday instead of monday

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