This is the day that is coming to destroy all the Philistines.
The reference to the day that is coming
makes it clear that it is the time that the Lord has determined (see Jeremiah 46:10, Jeremiah 46:21).1
All the Philistine includes Tyre and Sidon. These are two Phoenician cities to the north that were probably in some kind of treaty with the Philistines. These events are happening because, The Lord is destroying the Philistines, the remanent of the coastlands of Caphtor.
Caphtor refers to Crete, which is where the Philistines originally came from (see Amos 9:7).2
4 because of the day that is coming to destroy all the Philistines, to cut off from Tyre and Sidon every helper that remains. For the LORD is destroying the Philistines, the remnant of the coastland of Caphtor.