“you did not come during the appointed days.” Here Saul makes a weak attempt to explain away his error. As Robert Bergen notes, Samuel “did in fact arrive on the seventh day.”a There are a number of incidents in Saul’s life before he was rejected as king (1 Sam. 15:23-28), that show Saul was unfit as king, and his lying about Samuel to cover up his weakness is one of them.
“and that the Philistines were assembling themselves together at Michmash.” This can be seen to be a hollow excuse, because Saul is at Gilgal in the Jordan Valley and Michmash is about 15 miles away and 3,000 feet up in elevation, so the Philistine “threat” was at least a day’s march away.