The Feast of the Assumption is just that, a FEAST day.
Holy Days of Obligation are designated SPECIAL feast days, and they are never days of abstinence or fasting. The Little Flower (St. Therese of Lisieux) once was asked which was her favorite feast day, and she replied that it was Sunday. She always looked forward to Sunday as a specially enjoyable day.
So what's the problem with having an ice cream cone on a feast day? You didn't have five ice cream cones, did you? You didn't have a full meal and then top it off with an ice cream when you had no more room left, did you?
Come to think of it, I'm going to have an ice cream, myself, after reading your post!