I believe it was Easter. Remember Albert and Grady were painting eggs with the kids before they went off to the juke joint together, which gave Ceilie and Suge the time to find the other letters. What I didn't understand was the year. I might be wrong but I think the postmark for the letter was a whole year earlier. I'll have to go back and watch it again to be sure.
All typos and misspellings courtesy of a public educational system.