Rashi: It will be when their armies come against Yerushalayim.

Radak citing Targum Yonasan: He will invite them to a meal - "v'Ha'alisicha mi'Yarkesei Tzafon v'Havi'osicha Al Harei Yisrael" (Yechezkel 39:2).

Malbim: This explains how Hashem will gather the Nochrim to Yerushalayim in the days of Mashi'ach to judge them (mentioned above, 24:21). It is as if they are invited to a Mishteh. Based on the guest, it is "Mishte Shemanim" or "Mishte Shemarim"; refer to 25:6:2:3.

Rashi: They think that it will be pleasant for them like oil; it will be converted for them to "Mishte Shemarim" (dregs).

Radak: This refers to fat. It is a metaphor for the cup [of punishments] that they will drink there - "Ki Ka'asher Shesisem Al Har Kodshi Yishtu Kol ha'Goyim" (Ovadyah 1:16). I will make the wine "Saf Ra'al l'Chol ha'Amim" (Zecharyah 12:2, i.e. poison).

Malbim: It is a Mishteh of good, fat things. This is for those who will recognize the true creed and return to Elokei Yisrael. For those who stiffen their necks and refuse to go in the light of Hashem, it is Mishte Shemarim. There is no middle ground - either eternal life, or eternal shame.

Rashi: They are pleasant and fat like Mo'ach (marrow) of bones.

Radak: Memuchayim are very fat, like Mo'ach. When one drinks from a fat party, his heart is heavy and troubled, and all the more so if he consumes pure dregs without wine. This is a metaphor for the befuddlement that will be among them; they themselves will kill each other - "v'Alsah Yado Al Yad Re'ehu" (Zecharyah 14:13).

Rashi: They are pure dregs, without any liquid ¹ (wine or oil). All this refers to the war of Gog and Magog.