“After two days…on the third day.” This is an idiom meaning after a little while. Some of the Israelites were confident, as they should have been, that if they returned to Yahweh, He would bless them. It might not happen immediately, but they were confident it would happen shortly. Sadly, Israel never did return to Yahweh, and they were conquered by the Assyrians and deported from the land of Israel (2 Kings 17:5-23).
Anderson and Freedman write: “The use of the series x, x+1 to achieve a climax is common in ancient literature, especially in the Canaanite tradition” (Francis Andersen and David Noel Freedman, The Anchor Bible: Hosea). Normally the third day was used to mark a short period of time. Often it was connected with resurrection and life, which was due to the fact that it was believed that a dead body could be revived through the third day, after which decay made resurrection impossible (see commentary on John 11:6). Here, the people were saying God would revive Israel if they returned to Him.