Testing has shows that the cabalist sieges don't appear (or end) consistently. A difference of up to a couple minutes has been seen. If you have any insight, or see a pattern to the differences, please post on the forums. Thanks!

Cabalist Sieges

Current City Symbol Virtue Remaining Next City
Dolus
Temna
Nefario
Nefas
Avara
Indigno
Corpus
Fastus
There's more details about the astronomy workings on a page about time.