Question: Are Buddhists supposed to pray for the deceased on the 100th day after departure?
Answer: This is not a practice taught by the Buddha. It is a Chinese cultural tradition. The danger of clinging to having 100th day prayers is that some might become complacent or lazy, and waste time doing nothing useful within the first 49 days, during which rebirth usually takes place.
They might think that since there are 51 more days, they can take their time, thus missing the precious window period for guiding (开示) the deceased (who might still be around) with support-chanting (助念) to reach Pure Land, before the intermediate (i.e. bardo) state ends.
It is of course harmless to have more prayers on the 50th day… 99th day, 100th day, 101st day… death anniversary days and such, for dedication of more merits and expression of gratitude. However, these days have no spiritual significance for the deceased, while anniversaries are easier for the survivors to remember.