Midsommar, (or Midsummer, obviously, if you prefer the American spelling) is rooted in Nordic and Germanic pagan tradition, with rituals of fertility and youthfulness. It also coincides with St. John's Day on the Christian calendar. These days, however, it's all about enjoying the spring-turned-summer, and some really good food.
In Sweden, this holiday kicks off the summer vacation season, and is often spent with family and close friends. Stateside, we don't often see too many neighborhood bbq's with kids dancing around a maypole, (by the way, you can find an awesome tutorial for making one here) and often, Midsommar celebrations are held at Scandinavian cultural centers, and rightly so. (Here is a fairly conclusive list of American Midsommar events compiled by Nordstjernan.) By the way, this year it is celebrated on June 21st.
Even Ikea does it up with a celebratory Smorgasbord, as as they always seem to do, they sell many of the components you might need to make your own version in their marketplace.
If you would like to read more about the history of Midsommar, here and here are some good resources with some background information, and even some traditional recipes that you might like to try out.
Speaking of food, I love that Midsommar celebrations aren't barbecues. As an American, barbecuing is what we do in the summer. But it is so fun, from a culinary perspective, to have an outdoor celebration that doesn't incorporate meat on a grill. I'm actually not even sure Americans are capable of it. But it is worth a try, because the food is fabulous, and it is well worth venturing out onto a Northern European limb for such fare.
Here, here, and here, are some other fantastic places to find a variety of Midsommar recipes.