lots of dates missing from my calendar but calendar is still there showing holidays and a few monthly dates.

Calendars sync to email servers. If you have recently deleted an email account or changed an email password any events that were tied to that email account are gone. Holidays are a separate, subscription calendar.

You can also check that all of your calendar sources are syncing. Open Calendar app and tap on Calendars at the bottom of the screen; make sure all calendars are checked.