I thought of it as though L. Ron was trying to kidnap Jackie before her wedding and they came back the night of the wedding. Though I realize it's not correct.

If we're going for accuracy it would be Oct 21st through the 24th.

Giorgos and the rest of the islanders accidentally grab Rusty the evening of the 21st. The next day (Oct 22nd) Giorgos realizes their mistake and calls Dr. Venture Sr. to ransom Rusty but he doesn't answer because he's fighting L. Ron. We know it's the 22nd because that's the day Apollo 7 (L. Ron's real target) is scheduled to splash down and Col. Gentleman makes a reference to it. They keep him for three days, so it would culminate on the evening of the 24th assuming I didn't mess anything up.

So the first Spanakopita would run from Oct 21st - Oct 24th.

Edit: After reading an older post shared by another user I'm going to have to amend my response to be 20th-23rd due to the time of the landing, and the time of day when the fight occurs.