conditional formatting...

Hi all, please could you help me with this problem I am trying to solve.

I have a spreadsheet with columns headed by date and the rows are payments in.from different people. What I am trying to do is that if a payment of 55 comes in on the 1st of the month the rest of the month gets shaded a different colour. If it is any other amount then it stays it's default colour.
As this sheet is for a whole year I was hoping I could do this in some way so that I could drag it trough the whole sheet. I can do it if I type it month by month I think.

I hope this makes sense.

Thank you in advance for any help you can give.