The biblical requirement is to perform the bris on the eighth day of the child's life - his actual “birth” day is counted as the first. As the Jewish day begins at sundown (Lunar calendar), if your son was born during the day on Monday, his Bris day will be eight days later i.e on the following Monday. If your son was born after sundown on Monday, his Bris day will fall on the following Tuesday (because in Jewish tradition evening is part of the following day). Performing the Bris on the eighth day (wherever possible) is considered of paramount importance in the Jewish faith and hence a Bris may be performed on Sabbath, Jewish holidays and even Yom Kippur. If there are any doubts as to the child’s health and his readiness for the Bris postponement is possible after consultation with a Rabbi and medical professional.