Update 4/29/19: The bug affecting printing in Google Calendar appears to be fixed.
Trying to print your Google Calendar but keep getting a broken print preview window? Try enabling the “Show weekends” option under the Day/Week/Month/Year dropdown menu. If you don’t, you may be unable to print your calendar from any view.
It looks like this is a server-side issue, since a 500 error is logged to the browser console when the print preview window fails to load. Hopefully, Google will release a fix for Calendar in the near future, as the issue has already been reported on the Calendar forums: