If you're using Google Sheets and you are trying to figure out how to put a pop-up calendar date selector inside a cell, it is not very straight forward. We are going to go through a set of steps to show you how.
Pop-up calendars can be used to make it easier for a spreadsheet's users to enter data. Pop-up calendar date selectors can also validate data to give the user a message or reject the cell's input if it is not a valid date.
The pop-up calendar comes as a result of applying data validation to a cell or range. To apply data validation, choose Data -> Data Validation.
Once you are in the options for data validation, all you have to do is change the criteria to Date. The other options can be left as is for now.
It may not look like it, but you're done. You only see the calendar if you double-click in a cell. If other people will be using your spreadsheet, you may want to indicate this so they know that a calendar is there.