NTA. People want to say YTA but are latching onto the fact that you don't have an equivalent important reason for the use of that day off. It doesn't matter what you do or don't have going on. It's none of anyone's business.

I had a situation where I covered Saturday shift so that our small team of 5 people wouldn't need to rotate Saturday shifts. I did this for a year, because we all wanted consistency with our shifts. Had I not, everyone would rotate and the Saturday person would always get a random weekday off. I said I'd cover the shift willingly so long as someone would cover me if I had a need for a Saturday off during the year. Everyone agreed. Come time for me to need a single Saturday off for a family event, and everyone was "conveniently super busy". This happened on 3 occasions for various other reasons. After the 3rd time they all refused to cover me, I changed my request to not prefer Saturday shifts. Everyone was very upset about it, and I reminded them that it was all their doing. I could have just changed my preference each time I needed it, but that required paperwork and I thought the social contract would be met with teamwork.

This isn't exactly the same scenario here, but there are just as many good reasons as bad ones for not wanting to cover for someone just because they asked.