I need advice...

So, I really love my job, I'm very lucky. I work with a great bunch of gals, that I truly enjoy each day. But occasionally they schedule a "girls night out". And I certainly enjoy spending time with them. But after a long week at work, sometimes I'm simply to tired. But I will usually go, because I feel obligated. I always have a nice time, yet sometimes I'm really not up to going. If I'm not up to it.. What should I do.. Suck it up its only a couple hours?