the real "wedding" already took place in Australia....so this is really one of those "receptions back home" for the people that didn't/couldn't attend what is basically what we know as a "destination wedding".....still a very important event for many, so it really shouldn't matter to a "boss" ....but it does take a little juice off it actually being a "wedding" that you are a part of or actually "in"....
sounds like a little lesson can be learned here that you maybe should have been a little more forceful with your "heads up" a few months ago or whatever.....does your company actually have an official way that you are supposed to "request time off"....if so, did you do that...?....sounds like maybe you didn't cause you say you just gave him the heads up and then you also mentioned that you could maybe work "earlier in the day" and then still go to the wedding, but then realized later that wasn't possible because of travel.....so if you just kind of mentioned it and then waited for your "schedule to come out" and saw you were on the schedule, you maybe didn't give a meaningful enough "heads up"...there may have been others that "officially" followed the company policy of "requesting time off", so they trumped you here on probably one of the more requested days off on the schedule....
you also seemed to mention that it is a part time job that was working almost full time hours....if you are indeed technically "part time" and getting those benefits....you should consider yourself very lucky.....many places don't offer benefits to part time employeees....so you being "part time" may be coming into play here as well, as other full time employees, probably asked for that day off as well.....
so with all that said here....I can see how it shook out this way....there is probably some shared blame here.....you maybe didn't do what you really needed to do, and your boss is kind of in a "I need somebody to work" situation, so you drew the short straw when things shook out....and they may be taking advantage of you a little bit, because they know your level of commitment to them and that you will just "deal with it" or accept it because of what the job means to you, and your comfort level, etc....
I wish you the best in whatever you decide....if you find another job great, but I know that is always scary and uncertain and nobody really likes change....sometimes there are some hidden benefits to a job, like your proximity/comfort level/etc that don't really have a price attached to it about how much the job is really "worth" to someone, but it sounds like some of those things are pretty important to you, which I can totally understand....but sometimes those hidden benefits also come with a cost....which in this case, the "cost" may be being told "sorry no you can't have that day off".....best of luck...