The ceremony skill gives a boost to the effective percentage of the next spell you start casting. For example, you may have the spell ‘floating disc’ at 10%, but if you use ceremony first, your next attempt might be as if you has 30% or more in the spell. Very useful for succeeding at magic you don’t know very well.

Ceremony can improve a spell’s effective percent above 100.

If you see the message “You feel the powers align.” you have critically succeeded at this skill. This doesn’t mean that your next spell is guaranteed to succeed however.

The ingame help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: Ch.
It uses 1 endurance points.
Many times one can call on their god's favor to assist
them in casting a spell. Certain ceremonies and rituals
may be able to bring the blessings of one's god on one's 
actions. Of course the gods are very fickle and their favor
is not always granted. Others even say that the gods are no
help at all, but that these rituals are only for the sake
of mortals to ease their minds. Either way, most teachers
of magic urge their apprentices not to forget that gods and
that the tiny blessings that might be imparted unto them are
worth the trouble to make these rituals habit before trying
to cast spells.