Rachel's middle name is Karen, and was 24 in the pilot and 34 in the finale (Birthday: May 5, 1970).
Monica's middle initial is E. (she buys "MEG" shares because they form her initials), and was 24 in the pilot and 34 in the finale (Birthday: March 22, 1970). However, in The One With the Ick Factor, Monica stated that she was 26. She also stated that she was 26, in a telephone conversation with her mother, during the very next episode. This was clearly a character error, since Ross's stated age in the pilot episode, was 26.
Phoebe was 27 in the pilot, and 37 in the finale (Birthday: February 16, 1967). Initially she believed she was born in 1968, but her twin sister Ursula revealed that they were born a year earlier. She has a middle name but does not know what it is, as Ursula sold Phoebe's birth certificate to a Swedish woman.
Joey's middle name is Francis and was 26 in the pilot and 36 in the finale. Joey's birth date has always been a mystery. In the episode "The One With Joey's Fridge", Monica says he was 13 in 1981 making his year of birth 1968. This is confirmed in "TOW Monica's Thunder" (season 7, episode 1) Chandler and Rachel remind Joey that he's 31. (Year of Birth: 1968)
Chandler's middle name is Muriel, and was 26 in the pilot and 36 in the finale (Birthday: April 8, 1968)
Ross' middle name is Eustace, and was also 26 in the pilot and 36 in the finale (Birthday: October 18, 1968)
Ross, Monica, and Rachel are all Jewish, but do not seem to practice (Ross did once try to teach his son Ben about the Maccabees). Joey is a non - practicing Roman Catholic. Phoebe alludes to a variety of spiritual views but makes no clear declaration of beliefs. Chandler is apparently non-religious.
Non-practicing Jews celebrate Christmas all the time as an American tradition rather than one of faith, and the Gellars are not "part Jewish". In "TWO the Birth Mother", when Monica has been mistaken for a Minister, Chandler says "And you're Jewish!". Monica nods in agreement. If their Mother is a Jew and their Father is not, they - Ross and Monica - are Jews. The Gellars are non-practicing Jews. There is a bunch of confusion still as to Rachel's religion:
"The writers on the Friends DVD commentary certainly say they wrote her that way. But the character herself is markedly mum". http://www.jewornotjew.com/profile.jsp?ID=471
(If Rachel's Mother is Jewish, again, she is Jewish)
In Season 1, "TWO George Stephanopolous" Ross says his birthday was "seven months ago", and he says that today is October 20th, putting his birthday somewhere around March.