Then you either accept or politely decline. If something comes up (be it a better offer, a night spent on the sofa or something good on the tv) and you are unable to attend you get in touch with the host to say you won't be able to make it.
I have just hosted a milestone birthday party had a fabulous time but since \i got home I realised that lots of people didn't turn up. Hmm can I class myself as a grumpy old woman now and send them cross messages? 