A few years back, I did a research to see if it is possible to complete the promotion quest in the Altar of the Moon before you complete the promotion quest in the Alatar of the Sun. (
That is, promoting a druid directly to Arch Druid.) The result I found was a little bit confusing: while it is possible to complete the promotion, the experience point gained by every characters are not the same, a druid seem to be getting more experience point than those earned by a priest or an archer.
I repeated the test a few days ago and I find the following results:
1. Crusader, Archer, High Priest and Wizard all get 40,000 experience points from this "early" promotion.
2. The quest can be completed in any mid-night (not just the 15th). So it is a good way to get a promotion at the early part of the game.