I want to write a formula that works out what the crossword number in a daily newspaper should be.

For example, say the crossword number for today is 4000 and today is Friday.

What formula would return tomorrow's (Saturday's) number (4001) correctly?

And - most crucially - what formula would return Monday' correct crossword number - 4002 - as there is no daily newspaper on a Sunday?

Help very gratefully received. I'm going round in circles...