I'm having the same issue. It completely cleared the one I had been making, and nothing has made it come back for me, nor can I make a new one on any world, either.

Hey, thanks for reporting. The workaround for this is to first create as many tabs as you need for months/weekdays and then fill them so they don't get cleared - but we are currently working on the next version of Calendars which will a lot more stable, so keep an eye out for this update soon!