The Order assumed that Voldemort would expect Harry to stay with his blood relatives for as long as possible, so they confunded Dawlish to leak a false plan to Death Eater spies within the Ministry, so that they would think Harry would be picked up on the 30th. They had hoped that the Death Eaters wouldn't be ready to intervene on the 27th, or at least not with all seven Potters.

Well…the order chose that date,and they want to break Lily’s protection for Harry earlier.

But how did Snape know as the Order didn't trust him.

The entire idea was Dumbledore's. He told Snape to confund Mundungus into proposing the idea as his own.

So that exact date? It doesn't say that Mundungus came up with the date just the idea of the decoys.