Is it selfish if I request vacation time with family if it's during a busy time at work?

I rarely ever use any of my vacation days. My sister and her husband are planning a big family trip to Cabo for their birthdays in August.

One problem is that this is during my busiest time of year for my role where I prepare to issue hundreds of contracts for faculty and also it lands on a Friday where I do orientation day. Basically I have a ton of guest speakers from different departments talk. I don’t do anything other than introduce them. Orientation is 4 hours. We only do two orientation days per year which makes no sense because employees there for 6 months will be attending; so we are actually trying to do away with it by doing an electronic interactive onboarding orientation. We have started planning with IT but not sure if it’ll be complete by that time, we doubt it.

I’m sure they can work it out if I do even get it off. My coworker was in my role for 7 years before being promoted. Plus my boss and her have held several orientations in the past so it’s not like no one can do it.

I can’t ask my sister to push it back because other employees have time off other days and I can’t have her plan for later because I am going back to school for my MBA.

I really want to go and everyone else is. But I would feel bad leaving work when it gets pretty busy.

Is it bad if I even try requesting time off?