I don't have a calendar, and I'm going out of town for the weekend.
If the 4th of July is on Monday then we are coming back, and I can come over.
Great! See you for the 4th of July.
No one checks the calendar to see that the 4th of July is on Sunday. Janice comes back Monday morning and calls Jessie about coming over that night but Jessie is very mad because Janice didn't come the evening of 4th of July. If they would have checked the calendar Janice could've said she wasn't going to be there Sunday night and Jessie would not have been angry.