I'm thinking of booking the same holiday destination as my friend's anniversary
Does your friend know when you're planning for the vacation?
Not yet do you think it would be weird?
It is strange to join a friend on their anniversary. You should respect peoples boundaries and get your own trip.
Why can't I tag along, I'm a good friend
An anniversary is meant to be a celebration for the couple. It would be intrusive to show up during their private time on vacation. It might harm your relationship with your friend if she resents you were interrupting their couple's vacation.