Calendar!
Write the missing dates on the calendar.
May
For the grown-up: the answers appear on a second page under the sheet. They are not printed unless you choose Print with answer key under More.
A page of a month with some dates missing: write them in. Any month, a few or all dates. This one is “Calendar · May · Half missing” with 1 questions, and the answer key prints on its own page. Print it free in color or black and white, on US Letter or A4, or press Make your own to change it.