AITHA for being frustrated about my family’s conflicting plans?

My wife and I are expecting our first child in June, and are throwing a baby shower in April. We specifically chose a venue that was centrally located (closest to family in our home state), affordable, and available on a Saturday because we live in different parts of the country. The venues only opening was on Saturday the 12th.

Both my wife and I agreed to call our parents and validate their availability before we called the venue. When initially asked, both sets of parents agreed that it was a reasonable date.

Later that day, one of my parents tells me that that day actually doesn’t work for them, because they and my twin sister’s family (wife and 2 kids), and our extended family (cousin and her husband and two kids) were planning on going to the zoo, but offered for us to join them at the zoo that day and said that we should reschedule.

Some added context that I think explains their mindset is that my cousins are currently living abroad, and infrequently visit.

Am I the asshole for thinking that it’s a bit ludicrous for them to miss an event like this, or to suggest I reschedule for the zoo? Genuinely would like some perspective.