It isn't needing an excuse to drink. It is like 420, you can pretty much smoke any day you want, but it is nice to have a day where you know that you will be in good company.
And to the thread creator, you gotta give drinking more than one chance man, my first time getting drunk I was in this ghetto ass party I had no business being in, and I was flirting with a large and in charge mexican chick (wasn't even cinco de mayo either). My point here (other than god bless my friend for getting me the fuck outta that house before things got all sorts of strange with the senorita) is that your first experience with alcohol isn't always great. And, even if you still don't like it, don't hate on others for sippin some Modelo on cinco de mayo.