I'm running latest version of Thunderbird in Windows 10 OS.

When I select 'Calendar' and select 'Month' tab.Then close and reopen the Calendar, it shows the 'Month' tab. When I exit and restart Thunderbird, the Calendar auto opens on the 'Month' tab regardless of whether I close the Calendar or not. Providing the 'Month' tab is selected prior to closing the Calendar or closing Thunderbird.

So the action you are looking for already exists as default.

Do you have any programs on your computer that 'clean up' files such as CCleaner or even some Anti-Virus products can do this ? It is possible these kind of programs are removing files such as session files etc and so preventing Thunderbird from knowing what was last selected. If you have programs that can 'clean up' files, please make sure Thunderbird folders are exempt from scanning.

When I close and reopen the calendar it works perfect but when I close Thunderbird and restart it, it's not working. I have no programs like CCleaner and Bitdefender free edition as Anti Virus program.

I noticed the failure to retain the view with my default profile, but then tried it with a new profile and the view selection is remembered between TB restarts.

Going back to the default profile, I opened Tools/Options/Advanced/General/Config. editor and right-clicked the pref. calendar.ui.version, Reset, clicked OK to close Options. Upon restart, TB remembers the view between TB restarts, whether it be day, week, multi-week or month.

Thanks sfhowes. I tried this but every time when I restart TB the calendar.ui.version jumped again from the standard position to an adjusted position. It gives there in Dutch: aangepast (adjusted) (under status), integer (under type) and 3 (under number) instead of standard (under status), string (under type) and nothing (under number). So the reset I did is changing back with every restart of TB

Yes, the preference value does switch to 3 and stays that way, but the effect of doing the Reset is to make TB retain the calendar view, i.e. day, week or month, between TB restarts. The same thing happens with a new profile. You can create a new profile with Profile Manager and try it for yourself if the reset in your current profile doesn't make the view (not the preference) stick.

To start PM, close TB, Winkey+R, type thunderbird.exe -p

The computer can't find thunderbird.exe -p even if I give in the path to the thunderbird.exe file.

Winkey+R opens 'Run' window. Or type 'Run' in the search and select the 'Run' app.

Confirm you are typing this in the 'Run' window and not the search. Confirm you are typing: thunderbird.exe -p That is a space after exe, before the -p.