How can I be in the mood for something coming up (fx a party, a social gathering)?

Whenever I am up for something like a party, a social gathering, an event of some kind, I feel down. I don’t want to go and I would rather stay home and do nothing. Tonight I am going on a small excursion with some old friends, but I don’t feel like it. How can I remove this bad feeling. It’s kind of like I’m feeling home sick or just have a lack of energy. Does anybody have any advice on how to set your mood for something and be up-beat and free from worry?