What caused this Ravnica uncommon to jump in price in early 2015?
What's limited a card like Congregation at Dawn is that, unlike Worldly Tutor, it's split between two colors. That being said some Commander decks have been using it to set up things like Descendant’s Path and Garruk's Horde, as well as other cards that care about creatures being on top of your deck.
Mayael the Anima, for example, is a Commander that very much cares about the creatures on top of your deck. While she can only play one of those creatures with her ability, you can play Congregation at Dawn on the end of your previous opponent's turn to draw a creature that you would want in your hand instead such as Eternal Witness or some other utility creature. You can also use it to guarantee up to three targets for an endgame card like Genesis Wave.
Congregation at Dawn rose from a one dollar uncommon to around 3 dollars early on in 2015. This is probably partly due to any useful card with a converted mana cost 3 or lower being bought up due to the exploding popularity of the Tiny Leaders format. But as a lot of powerful creatures have been released in the Tarkir block, creature tutoring has become more important and powerful than ever.
If you're running a creature heavy Commander deck in green and white, this is a great complement to Worldly Tutor. As it's an uncommon from the older and wildly popular Ravnica: City of Guilds set, supply is very limited at this point. You'll want to pick up your copies today.