Macro for message box for unavailable dates

I am creating a schedule that currently has dates for several months across the top columns and several employees/contractors on separate rows that are on a rotation schedule and the days they are scheduled to work are highlighted below these date columns and a "LV" and "AR" is also entered when they fly out and arrive.

I would like to have a message box pop up if the date selected they leave falls on Sunday, Tuesday, Thursday or Friday as there are no travel available.

The problem I have is the dates are at the top across the columns on the top row and the names are on the rows below to the left and not sure how to write this.

Hope I am explaining this enough that someone can help me out. I have started writing any code for this as I don't know where to start.

Thanks.

Post a sample copy of the workbook - just enough anonymised data to illustrate your problem...