The One with the Monkey - S1-E10
Corrected entry: This episode is taking place the week before New Year's Eve, so why is Joey still working as an elf in a department store? During one scene, Monica is dismantling the Christmas tree and Joey walks in wearing the elf costume and talking about the single mom he met while she was waiting in line with her kids. These dates don't seem to add up.
Correction: A week before New Year's Eve is Christmas Eve, they are exactly seven days apart. It could also be on Christmas Day. Joey would still be working as an elf because little children want to see Santa on Christmas. Monica would be taking the tree down because she's obsessive about cleaning and the Christmas celebration at her apartment was over.