I just accepted a job offer (sent the offer letter a few mins ago) and my start date is mid-May (either the 11th or 18th). However, I have a pre-existing commitment: my sister’s destination wedding in late June. I need to take 5 days off for travel and the ceremony.

I really don’t want to be "that person" who asks for a week off immediately after starting. Since I already sent back my signed offer letter, what’s the best way to bring this up to my manager without looking unprofessional or unreliable?