The chief activities include drinking beer by the bonfire; grilling meat; eating the traditional Midsummer cheese (with lots of cumin!) and singing and dancing in the open air.
In these events, going to sleep is taboo, as you are supposed to greet the sunrise in the morning.
Many Latvians will be leaving the capital for the countryside. While it certainly doesn't mean there's nothing to do in Rīga (there is always an extensive official program of events in the capital), it does mean roads are likely to be clogged, which won't be helped by the police, who are conducting breathalyzer raids well before the actual party. So it's best to depart well in advance.
Those staying in Rīga will see the greatest parties held on the November 11 embankment and the Dzegužkalns hill across the River Daugava.
While it's understandable foreigners don't know traditional Latvian songs, singing along to the Midsummer songs' refrain Līgo, Līgo! will help you get into the mood.
You can make your own traditional wreaths of oak leaves (for men, especially those named Jānis) and meadow flowers (for women, especially those named Līga), or head to Old Rīga where you can stock up on your Jāņi wares at the annual Grass Market market near the Dome Cathedral.
Should you find yourself outside the capital and not really knowing what to do, there are always open-air parties held on outdoor stages in your nearest town.
It's a good idea to have mosquito repellent on hand, and if you've forgotten to stock up on cases of beer, these are sold pretty much everywhere, including in gas stations across Latvia.
It's possible someone may invite you for a walk into the fields of forest in search of the highly prized and rarely-seen "fern flower". This is a real honor, but don't be surprised if you discover something else entirely.
It's likely you'll be dared to jump over the bonfire. Maybe you'll have the nerve to follow Latvians in this charred adventure -- but safety should be your main concern; the same applies to intoxicated swimming and driving under the influence, both of which are a complete no-no.