Albert Barnes Commentary 1 Samuel 31:13

"And they took their bones, and buried them under the tamarisk-tree in Jabesh, and fasted seven days." — 1 Samuel 31:13 (ASV)

Under a tree - This should be translated as “Under the tamarisk,” a well-known tree at Jabesh that was standing when this narrative was written.

They fasted seven days - They did this in imitation of the mourning for Jacob, giving full honor to Saul even though he had fallen.