I'm really naive and ignorant when it comes to wedding questions and etiquette. Which is probably one reason it's good for me to stick around here

My friend's wedding is quickly approaching, and she's getting married on a Friday afternoon, so I have to take the day off work for it. She just mentioned on the weekend that they're planning on decorating the church on the Thursday morning, having lunch and then having the rehearsal in the afternoon... I assumed it would be in the evening. They said they still had to work some things out, and one of our other friends made a comment about how she'd need the day off work then.
Here's my question: is this something I should be taking the day off work for?
I work for my grandpa, and it's his business, so I know I will be able to take the day off work. The problem I have with it is that I'm already taking some time off next week, and for the wedding and I feel badly taking time off when I know I could be used at work. As well, if I take the day off I don't get paid for it. And when I'm only working during the summer, it's fairly significant to miss a day I could be paid for.
So how terrible is it for me to miss the decorating/rehearsal?
Secondly, is this something I should bring up now or mention it when she's officially made plans and lets me know about them?
I'm worried that if I say anything I'll look like a jerk who just doesn't want to be involved, since the other bridesmaids don't seem to have a problem with it. One girl even made a comment about us taking the week off work for the wedding and I was like, "what?!"