Context—So my coworker and I are technically not allowed to be on vacation at the same time due to us being the only two people in our department, and no one else can do our jobs effectively. 

I told her ahead of time that I would be scheduling a vacation on the third week of April due to school vacation for my brother. she told me that she was going to do so too, but would defer to my vacation since I mentioned it first. She emails our manager stating as much.

Today, she told me that she was going for 3 days + the weekend preceding but told me to double check which days I am off. I told her I would be taking 3 days plus the weekend after, as well as leaving a little bit early the day before my official vacation start to catch a late afternoon flight. I emailed my manager and told him that I will be leaving only about an hour’s worth of work behind for her.

So now she has to cut her vacation short by a full day.

TL DR—scheduling conflict caused my coworker to cut her vacation short by 1 day. AITA?