Albert Barnes Commentary 2 Samuel 17:1

"Moreover Ahithophel said unto Absalom, Let me now choose out twelve thousand men, and I will arise and pursue after David this night:" — 2 Samuel 17:1 (ASV)

This night - This refers to the night of the day on which David fled and Absalom entered Jerusalem. Ahithophel’s strategy was to fall upon David by surprise and, in the initial confusion of the surprised army, to seize and kill only David.