You probably don't need this answered since it's been posted over a year. But anywho...
In S1E21 'A Trip To The Dentist', Logan says to Aaron: "February," "Wow, you got a vowel right" and "Is this [birthday] party nine months early or three months late?" this puts his encontro, data of birth anywhere from January to March.
According to Logan Echolls' official myspace page found here:
link (PS this site is incredibly interesting.) Logan is a pisces.
Since people with the pisces sign are born between February 19 - March 20
por process of elimination Logan cannot be born in January and Aaron clearly got February wrong, therefore his birthday would have to be in March.
If this doesn't help you anymore, at least you can look into his smoldering eyes ;)