They sacrificed sacrifices to Yahweh, and offered burnt offerings to Yahweh, on the next day after that day, even one thousand bulls, one thousand rams, and one thousand lambs, with their drink-offerings, and sacrifices in abundance for all Israel,
So David is, has told of his own commitment and now he's challenging them for a commitment of giving to God. "This is what I have done. This is what I propose to do. Now who of you are willing to offer your service unto the Lord?"