I had checked the promotion quest for Great Druid and Arch druid and here is the result:
1. The quest for the promotion to the Arch Druid can be completed at any midnight, even before you are promoted to Great Druid.
2. The quest for the promotion to the Great Druid can only be completed on the 4 special midnights.
3. However, if a character completed the quest for Arch Druid BEFORE he is promoted to Great Druid, he will become Honorary Arch Druid even if the character is actually a Druid.
In other words, if you can handle the monsters, you can be promoted to Arch Druid at really early stage of the game.