It's a lovely dress (I have something similar), but even with the dots - it long and white. I would avoid wearing it to a wedding - you may get a few strange looks from people.
I'm sure there will be another non-wedding occasion to wear the dress, as I said I have something very similar (but a little shorter), and have worn it out for a few nice meals with my friends/husband. In fact, I've just remembered that I wore it the night before my wedding (we stayed at the hotel with a few family members, and all had dinner together in the restaurant that evening - I had already worn it a few times before this, and so didn't buy it for this reason) - I remember I chose to wear it, as it was white, but not a wedding dress, and so would be suitable for a bride the night before the wedding. With that in mind, I definitely wouldn't wear it as a guest!
The wedding isn't until May, so you have plenty of time to buy something else.