Albert Barnes Commentary 1 Kings 20:22

"And the prophet came near to the king of Israel, and said unto him, Go, strengthen thyself, and mark, and see what thou doest; for at the return of the year the king of Syria will come up against thee." — 1 Kings 20:22 (ASV)

Go, strengthen yourself ... - That is, “collect troops, raise fortifications, obtain allies ... take all the measures you can to increase your military strength. Do not be rash, but consider every step carefully, for a great danger is impending.”

At the return of the year - That is, “When the season for military operations comes around again.” The wars of the Eastern monarchs at this time, like those of early Rome, were almost always annual incursions into their neighbors' territories, beginning in spring and ending in early autumn. Sustained invasions, lasting over the winter into a second or third year, are not found until the time of Shalmaneser (2 Kings 17:5; 2 Kings 18:9–10) and did not become common until the Median and Babylonian periods.