You can wear store-bought Halloween costumes any day of the year except on Halloween. On Halloween you must wear a costume that is handmade.

Drinking Swedish glogg and eating Swedish pickled ham, Swedish meatballs, lutefisk, pickled herring (not me), and sylta on Christmas Eve.

Picking, growing, or buying one or more flowers, then, on May Day, placing it/them on someone's doorstep, ringing the doorbell or knocking on the door, then running away or hiding so that the person answering the door doesn't know who left it/them.

I know you said it can't be one we started. Well, I'm going to say one I started anyway. I would never let my kids eat cereals in which sugar was the first ingredient listed. However, on St. Patrick's Day, my kids would be given a box of the cereal Lucky Charms--the only day of the year they were allowed such a sugar-laden cereal. They're grown up now and STILL associate St. Patrick's Day w/Lucky Charms.

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Communication is the key.