Me and 3 friends have planned a weekend getaway for a BSB concert in August. The concert is on Saturday, and 6 hours drive time away. I work M-F and am off weekends, so I requested Thurs, Fri, and Mon off of work so that I can pack Thursday, drive Friday, concert Saturday, drive home Sunday. I took Monday off to relax at home before work again. I put in this request at the end of November and my Supervisor approved it. Now, my department is made up of just me and my supervisor at the moment, as work is slow and corporate has recently deemed we didn't need to hire a replacement for a co-worker that left in October. We are the only 2 people that can do what we do, but have been able to manage with just one of us for recent vacations.
This past Monday, my supervisor says to me:

Supervisor: I forgot to tell you! I hate BSB but I'm going to one of their concerts.

Me: Oh, nice. Where?

S: Hershey Park.

M: How funny, that's the day after my BSB concert. Do you plan on going to the park?

S: maybe saturday. My friend really loves BSB and was sad tickets were so expensive nearby so this weekend her and some friends decided to buy tickets to Hershey because they were way cheaper. And I decided why the hell not, and have a girls trip. My friends got floor seats for around $300 which i didn't want to pay so I got seats in the 300 level for around $50, but I'll walk around to visit them. I told the manager that I'll be needing that Monday off.

M: Oh, well I already requested that Monday off and you approved it, so...

S: I'll be 4 hours away so I don't know how I'll be back in time for work Monday

M: I'll be 6 hours away, so... *blink, blink*

S: I'm sure we can figure something out by then. It's 4 months away. 

So after that little exchange, I made sure to go to our manager and explain that the day my Supervisor told her she needed off, I had already requested and was approved for. Just in case things couldn't be "worked out."

AITA if I refuse to give up my Monday off because my supervisor decided to schedule a trip and buy tickets before making sure the day was available?