The one with all the birthdays

One with two parties (Season 2: airdate, 5/2/96) --

One where Chandler can't remember which sister (Season 3: airdate 1-9-97) --

One where Rachel smokes (Season 5: airdate, 4/8/99) --

One where they all turn thirty (Season 7: airdate, 2/8/01) --

One with Phoebe's birthday dinner (Season 9: airdate, 10/31/02) --

One with the cake (Season 10: airdate, 10/23/03).