Answer:
   
   The circumcision is done the eighth day after birth, unless ill health
   or serious medical problems prevent it. Even Shabbat does not stop a
   Bris. If such a child is not circumcised, he is nevertheless
   considered a Jew [San. 44a; Hoffmann, Melamed Leho-il, Yoreh De-a,
   #79]. However, if there are no medical contraindications (e.g.
   hemophilia), it is incumbent for the individual to arrange for their
   circumcision when medically safe to do so.
   
   There are quite a few customs associated with circumcisions:
     * In the Ashkenazi community, on the Shabbat night (Friday night)
       prior to the Brit, the community comes to the home of the newborn
       to welcome him with singing and thanksgiving to Hashem on his
       birth, and a small meal is served including chickpeas (ar'bes).
       These are served as a sign of mourning: the child mourns that the
       angel caused him to forget everything he learned in his mother's
       womb (just one explanation of many for this custom of eating
       chickpeas).
     * In Sephardi communities, the night before the Brit is called the
       night of "Brit Yitzchak" and the community and family gather to
       learn the "Zohar" together, to sing special songs and have a
       dinner. In many places people from the community and family get
       together and study all night not only on the night before the
       brit, but also during the preceding week.
     * The greeting said to the newborn when brought to his bris is
       "baruch haba"--blessed be the one who is arriving. It's a very old
       greeting, dating back to the workhands' reply to Boaz when he
       greets them in Ruth. The origin is probably a blessing God
       promises the Jewish people if they observe the Torah (Deut 28:6).
       "Blessed you shall be when you come, blessed you shall be when you
       go." It is part of a general covenant about getting the land of
       Israel, listing blessings and curses that will visit the land and
       the Jewish people depending upon their observance.
     * One does not issue invitations to a brit. This is because it is a
       mitzvah to attend a brit, and one tries as hard as possible to not
       refuse the opportunity to do a mitzvah. If you are invited and
       refuse, you run into this problem. If however, you are only
       informed of it, you have not been formally invited and circumvent
       the problem. Furthermore, since Eliyahu (the Prophet) will attend
       (the chair in which the holder of the baby sits is called
       Eliyahu's Chair), it is impossible to refuse an invitation.