Combining more than one "IF" function

Hello,

I wonder if anybody could please help me with this. What I'm trying to do is as follows.

If cell A13 says "Sunday" in it, then I want F13 to display 13, otherwise 9. No problem getting this bit, just used =IF(A13="Sunday",13,9). What I also need to add is that if A13 = Sunday AND B13 = 01/12/09 or 02/12/09 (or any other day until Christmas), then F13 = 18. But if A13 does say Sunday but isn't a day in December, then F13 = 13. And if A13 doesn't say Sunday and nor is B13 a date in December, then I need F13 to just display 9.

I assume all this can be done with one very long IF function. I just haven't got a clue how to string it all together.

Many thanks in advance.