My friend is getting married in a month, what if I can't go?

One of my very best friend’s emails me today and tells me she may get married in October. My other two friends are like no problem, we can be there! I am worried because my boss said no way to to one of the potential days, we have a ton of people out that day and I have no coverage (I am the lead lab tech in a very busy emergency animal hospital). I told my friend that day might not be possible for me (she asked me to be honest), and i feel terrible. I would do anything for her, but I also do not have a job with the luxury of taking off whenever I want. I already took a week off in September! What should I do? I want to be a good friend, but I also have a demanding job.