The One Where Rachel is Late - S8-E22
Other mistake: Ross wants Rachel to change, but she claims that it is 100° out. Monica and Chandler got married on May 15, the day Rachel was officially pregnant. 9 Months after May would be Febuary. It would be cold out.
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.