The Full Moon in November, 1988 occurred on November 23. Using this as the FIRST Full Moon, the 100th Full Moon happened on November 25, 1996. The Full Moon on December 24, 1996 ended the 100th complete synodic month since November 23, 1988 and you would call it the 100th Full Moon SINCE November 23, 1988. The Full Moon on January 23, 1997 will be the 101th Full Moon since November 23, 1988.