A "Bar Mitzvah Monkey" is a prostitute who has been hired for a Bar Mitzvah. They are normally hired by friends of the male who the ceremony is for. These women are pre-paid to show up at the ceremony and fornicate with the male who will be journeying into adulthood. It is common for these women to pick up extra money by also selling to guests of the Bar Mitzvah. Not as commonly, these prostitutes may be hired for Bat Mitzvah.
"I'll be hiring a Bar Mitzvah monkey for Eli's ceremony this weekend."