I have read the other threads about this issue and have tried the following fixes: Disable Lightning and restart. Download and re-install Thunderbird. Delete and reinstall Lightning. When I go to View, the calendar selection is greyed out. I still have the calendar side-bar but not the option of viewing the calendar month by month and dates and reminders in the near future are available. Lightning is enabled and the latest version was installed from the add-ons page. This came up a few days after my last update which has been buggy in other ways. Windows 10, 64-bit.