Day Start simply means that the game starts with a Day phase.
The advantages to doing this over a Night Start are that:
- Everyone who signs up for a game gets to play.
- The balance of the game cannot be thrown off by Night 0.
The stated disadvantage of Day Start is that players do not have any information beyond what is in their Role PM. Whether this is much different from Night Start is debatable.
Most games now are Day Start.