'First Day' in the Bible

It happened in the eleventh year, in the first day of the month, that the word of the LORD came to me, saying,

It came to pass in the seven and twentieth year, in the first month, in the first day of the month, the word of the LORD came to me, saying,

It happened in the eleventh year, in the third month, in the first day of the month, that the word of the LORD came to me, saying,

It happened in the twelfth year, in the twelfth month, in the first day of the month, that the word of the LORD came to me, saying,

"'Thus says the Lord GOD: In the first month, in the first day of the month, you shall take a young bull without blemish; and you shall cleanse the sanctuary.