Transferring previous data and new data together into a new sheet in the same cell

Hello. I'm attempting to make a data entry sheet calculating what days the latest entry was made, each sheet is a month. The data needs to be transferred to the next month while holding the information/date of the previous month until new data is entered making the formula update to the new date of latest entry.

Any help would be greatly welcomed and appreciated, I'm pretty stuck on this. Thank you.

Alternatively, if anyone knew a formula for getting a last entry date from across multiple worksheets that would be equally as helpful.
Current formula is =LOOKUP(2,1/(C6:AG6<>""),C$4:AG$4)