It's still quite a while before I can apply but I've been wondering about the following issue:
I know I can apply 90 days before I fullfil the 3 years US residency requirement, here's my question.
Hypothetically let's say during the 3 years I went on a one month vacation trip outside the US and my greencard 3 yr anniversary would be May 01, 07. When can I send in the application? February 1 or January1?

Hope I didn't make it too complicated. Was just wondering if I have to take account for all this short trips outside the US.

Thanks.

the 1 month vacation is not a problem, you still can file within the 90 days before the third anniversary of becoming LPR.

A good read for this is: http://www.uscis.gov/files/article/M-476.pdf

Yes no need to worry about the 1 month, just might have to bring proof. So if your anniversery date is May 1st then count back exactly 90 days (usually send on the 89th day to be sure). So depending on leap years etc, you need to make sure you count in the 31 days for Jan and March and whatever Feb has as well. This is not 3 months, its 90 days...