Why does the woman refuse to go the party?

M: The Students’ English Club is having a party on Saturday night. Would you like to come?
W: I would like to, but I have to work at a restaurant on the weekend.

M: What’s the matter? You’ve been sitting there for ages, just staring into space.
W: I told the Browning I’d send them a postcard. Now I don’t know what to say.