When Lightning starts, it gets your network calendar data from Google. Any changes you make are sent to Google so that the two calendars are in sync. When you are offline, you don't have that access, so if you start Thunderbird offline, it has nothing to display, and no additions or changes would be updated at Google.

If you have long periods without Internet access where you need to view your calendar, you could export your calendar while online to a local file and then import it to a local calendar (such as Home) for viewing. The big problem with that is that any changes you make to the local calendar would not be propagated back to Google. At least you would have something to look at.

Thank you for your answer, Duggabe, but this is not quite the solution I am looking for. One cannot always know when they need to access the calendar data offline, and I guess no one is willing to export everything manually every time he/she makes some changes. I expect this to be automated. My factory default Android calendar lets me access and even edit my calendar data offline. Practically, a local copy of my calendar gets synced with Google, and that local copy can be edited any time. Numerous applications manage to sync local copies this way. I think this should be a high priority feature, and to be honest I expected Lightning to do it this way. Especially that there are many sites that guide you how to set up 'offline support'! Like https://www.alansitsolutions.com/blog/2014/06/26/adding-google-calendar-thunderbird/ But as I see, this feature is not available any more. If I choose to add a 'Google calendar' in the most recent version of Thunderbird, the option of 'Offline support' disappears. Which still does not offer offline editing btw, but at least you can access your existing calendar data. And if you go to https://support.mozilla.org/en-US/kb/changing-calendar-preferences you get the impression, that they are working on some 'Cache'. It is in the experimental phase yet, which is why I could not find this experimental option, probably because I am on the release channel. Anyway, I would be glad to know at least that this 'cache' option is intended to solve exactly what every Android user can already do, namely full offline access. I hope that it will appear soon in a stable release.

re : I am using Thunderbird portable 31.2.0 with Lightning 3.3.1

Perhaps you could ask the Thunderbird Portable support.

• http://portableapps.com/apps/internet/thunderbird_portable

Thunderbird Portable Support Forum:

• http://portableapps.com/forums/support/thunderbird_portable

Could you (or anyone) please confirm that the options I am missing ('Offline support' for Google calendar and/or (experimental) 'Cache' option among the calendar properties) are actually there (and working) in the installed version of Thunderbird? Meaning that the whole issue is only an issue with the portable version? Thanks!

In standard desktop Thunderbird. If been doing a bit of a search and it would seem that there is an offline cache available..

A person was using it when they had an issue.

• https://support.mozilla.org/en-US/questions/1025138

Matt says: when I right click on a calendar and select properties the dialog contains an "Offline support" option.

The bug mentioned in the link above discusses the topic using Google Calendar extension 1.0.1. But Philipp says it is ok in version 1.0.2.

• https://bugzilla.mozilla.org/show_bug.cgi?id=1081922

Philipp Kewisch [:Fallen] 2014-11-18 12:38:13 PST Displaying items when offline seems to work for me in 1.0.2.

More general info:

• https://wiki.mozilla.org/Calendar:GDATA_Provider

I have just installed the desktop version of Thunderbird, with Provider 1.0.2. and there is no 'Offline support' option for Google calendars. Everything seems to work the same way as in the portable version. I checked out the links you sent me (I don't know why I haven't found them before, but thanks anyway!), and I suppose that Philipp Kewisch' report on the working offline support means that when you go offline within Thunderbird, the calendar keeps working, which is the same with me. The problem is when you are already offline when you start Thunderbird. In this case, I experience the same as the others: empty calendars! Now I move to the other threads, because they seem to discuss the same problem. Sorry for starting a new, I couldn't find those for some reason.