Car and Symposium Trouble

I forgot to mention something yesterday. Two things on my car broke. And nothing on my car has ever broken before. I'm very good to my cars, except that I tend to spill coffee in the seats and sometimes I cook bean burritos on the dash if it's sunny out. I really do that. It's solar cooking at its best. I might share the recipe if I'm asked politely.
Here's the damage:
1. My gas cap snapped off the little plastic chain. This is minor but exceedingly annoying.
2. My car's check engine light came on. I am a firm believer in ignoring lights that come on in the car (as Big Bang Theory's Penny says, when alerted to her flashing check engine light, "Oh yeah. I've got to put some electrical tape over that."). But I also noticed a sort of thumping when I'm stopped at a light, in drive, foot on the brakes. I will have to bring my car in, and this is the first time ever for me except for repairs when someone hit my parked car. TWICE. (Two hit and run incidents in the space of a month!). Other than that, I've been blessed with a very low maintenance vehicle that has only needed oil changes in 5 years.
By the way, I drive a Scion xA, which is no longer made. But it's a great car. It is TINY but ridiculously ROOMY. You can seat 5 more comfortably than in many SUVs, and the hatch back allows for a lot of storage. I really like that the seat position allows you to sit up instead of reclining like many cars. I just like that better. And of course, it fits in small parking spots and gets good mileage. I think I just wrote all that to persuade myself that this car is worth the hassle of going to the dealer!

Last night my boss called me about this HIV symposium. It turns out that yes, I'm going, and this is great news for my job of course. There will be lots of networking opportunities and an enormous amount of information to learn. There is only one tiny problem. The conference starts on a Thursday night - the same night as my in-laws bought us tickets to Wicked for Christmas! I absolutely can't pass up this symposium - it's an honor for me to be asked to go - but I feel bad for my in-laws. I'm not sure how to handle explaining this to them. I'm sure they will understand but I still feel like an ungrateful wretch!