Dear Sir (Or, if we are now close enough friends that I may return your greeting of "Dude!" in kind, Dude),
I regret to inform you that I will not be attending your party tonight, July 7th, at ten p.m. Instead, I shall be working, and therefore occupied. My co-workers will not be attending either, as, depite your request, I will probably not bother telling them about this party; and if the topic does come up, it will probably not be in an overly positive light. Rest assured that should I somehow get out of work before this time, I will also not be attending, as I am somewhat averse to gatherings of people I don't know advertised by men in tight t-shirts drinking beer on the street at one o'clock in the afternoon yelling at random passers-by.
Thank you for your interest in spending quality time with me.