June 21 is our "longest day" in the northern hemisphere, with longest hours of daylight; it also marks the official beginning of summer and many places celebrate with a parade or festival. (If you're looking for information about the tilt of the earth's axis and the earth's revolution around the sun, take a detour to a summer solstice at About.com's Geography site.)
The events below are all general come-one-come-all celebrations with no religious tone. Note that some events -- such as parades-- may be celebrated on a weekend, not on the actual summer solstice day.
Summer Solstice Celebrations
Alaska - Summer Solstice Celebrations
Top billing in this list of festivities goes to places who enjoy 24-hour sun on June 21st. It's no surprise that in Alaska many cities and towns celebrate the Summer Solstice.
Yellowknife Summer Solstice Festival
Also "way up north", the capital of Canada's Northwest Territories has a multi-day Summer Solstice festival, with music, and face-painting and other fun for kids.
Summer Solstice in Scandinavia
In other Lands of the Midnight Sun: "Midsummer's Eve" is hugely popular in Scandinavia, and in Sweden, it's actually a national holiday. About.com's Guide for Scandinavia for Visitors overviews the occasion in several countries. In Norway, for example, bonfires are a tradition. Even ourists visiting Oslo are likely to find a bonfire near their hotel.
Moving to celebrations in lower latitudes, scroll down for celebrations in New York, Californa, and more.
England: Stonehenge and Glastonbury
Mysterious, mythic Stonehenge is deeply associated with the solstices, and our About.com Guide for UK Travel calls summer solstice a "truly magical time" there: "an ad hoc celebration that brings together England's New Age Tribes (neo-druids, neo-pagans, Wiccans) with ordinary families, travelers and party people." See more about Summer Solstice at Stonehenge. Meanwhile, nearby at Glastonbury -- a place known for legendary associations with King Arthur and the Holy Grail -- travellers will find a family-friendly Sunrise Celebration in early June.
NYC: Times Square Summer Solstice
In New York City's tourist hotspot Times Square, an annual event on the summer solstice day is (would you have guessed this?) a free yoga-fest. Check for other festivities at the New York City Parks and Recreation site.
(An interesting digression: New York City has a phenomenon called "Manhattanhenge". Just as Stonehenge has a moment at the summer solstice when the sun and the stones align just right, Manhattan too has magic moments when the sun "sets in exact alignment with the Manhattan grid, fully illuminating every single cross-street for the last fifteen minutes of daylight." These occurrences happen twice a year, in May and July.)
WI: Green Lake 10th Annual Summer Solstice Celebration
A day-long party with "Bed Races", a town-wide pajama party, music, games, contests, and more. Green Lake is within an 1-1/2 drive of Wisconsin's major cities, and is also a 3-hour drive from Chicago.
CA: Summer Solstice in Los Angeles
A round-up of events, including Summer Solstice at Haramokngna American Indian Cultural Center, and Summer SOULstice Festival in Santa Monica with kid-friendly activities such as scavenger hunt, pony rides.
Los Angeles: Equinox Festival
The California Traditional Music Society puts on this festival that features folk music, storytelling, family activities, dance, and more.
CA: Santa Barbara Summer Solstice Parade
This city between LA and San Francisco has a parade and a three-day festival, with theatrical and musical entertainment, and a Solstice Children's Festival on two afternoons.
WA: Fremont Summer Solstice Parade
Started in the late 80's in this Seattle neighborhood, this Parade is now a "citywide community art parade". Anyone can join in the parade, or picnic afterward in a park, with music and food.