This isn't an official ceremony, but it is very similar to one.
If a cat dies, a leader must announce it to their Clan for all their Clanmates that do not know. Usually they say a few words about the cat's life to remember it's contributions to the Clan. If the cat was evil, the leader says something like:
Because of the troubles (cat's name) caused (Clan), he/she does not deserve a proper vigil or burial. (Two or three warriors), bring his/her body to the edge of our territory ('the edge of our territory' can be replaced with 'carrionplace' if the cat was in ShadowClan) and leave it there for the (one or two scavenging animals, such as foxes or buzzards).