How to update a excel calendar from other sheets in the same workbook?

[eelino]

Hello,

I'm trying to find a way to update an excel calendar from various sheets within the same document.
A friend sent me this excel in the attached.
It works perfectly for 1 sheet but I couldn't figure out how to duplicate several of these sheets and have it update the same calendar.
In fact, I couldn't quite figure out how this wonderful excel works, I don't have any programming skills

Would any of you kind experts please help me?

Thank you so much!
eelin