“Now it came about in those days.” So this event is happening in the north-central part of Israel while David is making raids in the Negev in the south of Israel.
“the Philistines gathered their armies together into an armed force to fight with Israel.” This gathering occurred in Aphek (1 Sam. 29:1). From here, David was sent back to Ziklag (1 Sam. 29:10-11) and the Philistines marched north to Shunem in the Jezreel Valley (1 Sam. 29:4).
“know, yes, know.” The Hebrew text of 1 Samuel 28:1 uses the figure of speech polyptoton for emphasis. See commentary on Genesis 2:16.