First of all - that sounds like the best birthday EVER!
I have been assigned to the busiest committee on campus as a faculty member because they have to have X number of faculty, staff, administrators, students on all campus-wide committees. The one I'm on is tasked with very specific student issues dealing with disciplinary hearings and the like and they meet a LOT. Seriously? THIS fall, when I am teaching 2 extra labs and an additional lecture course that I have NEVER taught before. I will have my biggest course load I have had the whole time I have been here and extra responsibilities related to the 2 faculty who will be off and THAT is the semester they decide to put me on a committee? And it is the busiest one on campus? REALLY??????????????????????????? I am co-advisor for a campus honor society and my partner in that has a ton of medical issues going on right now which throws a lot of responsibility for that group on me plus I have to order all the supplies and do all the budget stuff for my entire department.
You know you COULD just tell them that you have all those responsibilities this semester and isn't there anybody else they could assign to that committee? They are probably not thinking (or knwing) that you have all of that other stuff on your plate.