I have also lost all my calendar entries but I am not sure if that was a result of an automatic update. I only have 1 calendar. I have tried to re-enter some events but they also disappeared after a restart. This problem started 3 or 4 days ago.

Go to events and tasks then import.

That got me half way there. It wants to import iCalendar(*.ics) file. I have no idea where this file resides.

If you previously used the provider got Google calendar, it has a bug and you will need to address questions about getting it to work to the addon developer directly.

There is a support link on the addon download page. https://addons.thunderbird.net/en-US/thunderbird/addon/provider-for-google-calendar/

With the release of version 91 options have been renamed preferences. https://support.mozilla.org/en-US/kb/new-thunderbird-91#w_options-renamed-to-preferences-under-windows-for-cross-os-consistency

I get the impression your trying to import something, but I have no idea what as you mention ICS files, but the calendar does not use them other than to initially get a new calendar and certainly not to sync to a web calendar. SO despite there being discussion here, I have no idea why you would be importing anything.

I have not saved or exported the calendar so I have no idea what to import or where from.

If your accounts were IMAP the calendar was and is on the server i.e. Google calendar

If your accounts were POP3 the calendar could be on the server (in this case as IMAP) or could be local on your hard disk. If you have no copy on your disk then recreate events.

Advice You must always keep backups of emails, contacts and events locally on your disk. In this way whatever problem arises the solution comes in no time.

This whole thing is a confused mess to me. After the upgrade to 91.0.3, I lost all of the data in my calendars. The calendars are there and enabled but blank. Since I have the Google calendar on my cellphone, I gather the data is there and is on the Google Cloud. How do import the data from the Google Cloud to Thunderbird on my PC?

Go to account settings and choose the one you want. Press synchronize and tick all options on the right panel

I am also on 91.0.3 and I have used the Thunderbird calendar for years so it had lots of annual dates such as birthdays as well as one off appointments etc. All the dates just disappeared and any new ones I enter also disappear after a restart. I have several e-mail accounts but only the one calendar and I haven't found it copied on any of the imap servers. So I am confused about the advice. I have not exported or copied the events or calendar anywhere and when I look at import I have no idea where to import from. All the e-mail accounts are all synchronized. I have no idea why this has happened and have spent hours trying to figure out how to implement the advice on this thread to no avail.

The author of the gdata provider addon which I am guessing you are using has as I understand it finally released a version of the addon that works with your version of Thunderbird. (1st September) https://addons.thunderbird.net/en-US/thunderbird/addon/provider-for-google-calendar/

I suggest you endeavour to update your addon by going to the addons menu in Thunderbird.

Thank for the information. However, there is a problem. The compressed file downloads in a format using the Ultra File Opener to extract. They want $50 to purchase in order to extract and save. What has all of this to do with Thunderbird screwing up my calendar files with the latest update. Maybe I don't want the connect with Google calendar any more and just happily use the calendar in Thunderbird. If I just delete that add-on can I use my calendar? Needless to say I an not a happy camper

@david36FL Your failure to understand what addons you are using or how to install them is why you are finding this most confusing. Did you go to the addons menu in Thunderbird? Have a look for the addon I linked to there.

My guess is you tried to directly download the XPI file. It failed to open when you double clicked it and then you searched to try and find an application to open the XPI with. Not really going to work.

Thunderbird opens XPI files natively from the addon manger, but prefers you to search using the name and just click install.

BTW I have never heard of this ultra file opener software until today and really after googling it I see little or no value in it at all. Other than to extract money from folks that is.

Perhaps read this article on how to install an addon. https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

My mistake in not understanding how to update the provider for google calendar. Since I had automatic updates turned on, I thought it would have updated but it didn't. I manually updated it. I still do not have any data in my calendars

Matt, Let's go back to the beginning and start all over. With the most recent Thunderbird update I longer had any events in my calendar and could not make new entries. I went to the developer's website and downloaded the updated version of Provider For Google Calendar. That did not change any thing and the calendar still does not work. I really need to get my calendar working. All my information is there in my cellphone Google calendar. What do I do next?

Do you use Google tasks?

I do not. I can install it if will help solve my problem. Do I need on both PC and cellphone? My Google calendar is working OK. It just doesn't communicate with Thunderbird.

After downloading the latest version of Provider For Google Calendar, check is the addon enabled, not sure but you may need to restart Thunderbird.

In 'Calendar' for the gmail - click on the 'Enable' again, then try right click on gmail calendar name and select to synchronise.

I have done this several time with no results.

Now that you are using the most up to date version of Provider For Google Calendar....Have you tried to remove the gmail calendar, restart Thunderbird and then recreate it ?

Well, that sort of improved things. I did as you suggested. I had 3 calendars Birthdays, Holidays, and dhallam@knology.net, with information in my Thunderbird and Google Calendars. I deleted only my Home calendar (dhallam@knology.net; a no longer valid email address calendar). When I restarted Thunderbird, the Birthday and Holiday were there. Both were Locked Read Only, and the only birthday was mine. I then re-added the dhallam@knology.net (what I am considering my Home) and got no information sync with my Google calendar on my cellphone. I did use my gmail.com address as it had been before for the new calendar. At least I could make entries in this new calendar.