I didn't know what that meant either, so I Googled it. A party favour (party favor - U.S.) is either (1) a small gift given to guests at a party as a gesture of thanks for having attended or (2) various drugs such as meth, ecstasy or cocaine when taken to parties, clubs etc. But I think that kissing under mistletoe (rather than kissing under party favours) is the traditional romantic gesture in Europe that was intended.