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!

I was worried that today would have the same prohibitions as a Sunday about buying and selling or servile work.

That wasn't clear before.  Our Lady came to La Salette to give the message that Catholics doing their business transactions on Sunday was a matter of grave concern to her because it's offensive to God.  We ought to sanctify our Sundays, not make them into a marketplace.

If you'd like to avoid the scruple, you could buy your ice cream on Saturday and keep it in the freezer until Sunday, next time. Of course, if Monday is a Holy Day, you'll need to think ahead and have TWO ice creams in the freezer Saturday.

Okay?

But if it's a spontaneous thing, you can buy at a shop on Sunday all right, but if you can avoid making a habit of just shopping on Sunday, that would be better.