If you have several days and add a new day it removes all information entered for days previously entered unless you save between each day.
Hey, thanks for reporting! We are aware of this bug and the workaround I can suggest is first opening the amount of tabs for months/ weekdays you want to use and then fill them (don't forget you need to click "Save Change" as Calendar don't support autosave right now). However, we are currently working on the next version of Calendar which will resolve this issue, please keep an eye out for an update soon.