3-mo calendar - starting on a particular date

Hello,

I need to create a 3-mo calendar (Sun-Sat) that other people can use.  The purpose is to show critical deadlines in those 3 months starting on the day the project is initiated.  Specifically, the calendar needs to populate 78 days from the first day.  So...Day 1 is, say, May 25, 2022.  Then - the calendar needs to highlight a key window between 28 days and 42 days after May 25.  Then the calendar needs to highlight a final day for completion - 78 days after Day 1.  Someone should be able to type in a date (May 25, 2022) into a cell - and then have the whole 3-mo calendar (not inclusive of that cell) populate with the info above, starting with Day 1.  It should be super easy for the person who needs to use it.

I've read and watched material online and, unfortunately, not finding good direction.  Any suggestions?

Thanks!

Nancy