We first meet janice in the episode "The One With The East German Laundry Detergent". Series 1 Episode 5. As Marwell7 already mentioned, we do not see him meet her. They have already been dating a while and he dumps her in this episode.
By the way, he then gets back with her for New Years Eve a few episodes later, dumps her, then they get back briefly a few more episodes down the line on Valentines day so those could be the holiday episodes you're thinking of.