The colder the country the more passionately they celebrate their summer. So it is with the Finns, who revel in months of sunshine and cider on the terraces as they shrug off winter’s grey blanket to catch a Nordic tan. The signs are small at first – markets feature strawberries or peas eaten fresh from the pod – but by Juhannus (midsummer) the nation has definitely defrosted.

Usually the third weekend in June, Juhannus sees Helsinki empty out as Finns head for their cottage in the Lakelands. Many cottages still enjoy traditional saunas right down to the wood fire that has best lashed with beer to give the steam the yeasty flavour of a bakery. No real Finnish sauna is complete without vihta, the bunch of fresh birch twigs that are whacked against the skin to release their sap. After the self-flagellation, it is time to start drinking your way towards Friday and Saturday night’s bonfire.

The first kokko (bonfires) of Juhannus crack into life around 9 pm with the lights of neighbour’s fires ringing the lakes. Before Christianity came to Finland fires were lit for Ukko, the Finnish god of weather and crops, and the holiday was called Ukon juhla. The greatest of the bonfires was called Ukko-kokko, honouring the god who would bring great harvests when summer came to an end. Swedish-speaking Finns harken back to older pagan days by erecting maypoles (midsommarstång in Swedish) though the festival was re-branded for John the Baptist (called Johannes Kastaja in Finnish) banishing any unholy ghosts.

But Finns still celebrate with plenty of spirits. A couple of post-sauna beers are compulsory though many prefer a quenching drink that mixes gin with a grapefruity tang, one of the country’s most popular summer tipples. And you will need a good drink, because as the glow of the bonfires dies down the bravest (and drunkest) Finns will swim across lakes to bonfires where the party still rages on.

In Finnish folk legend, midnight marks the time when unmarried Finnish try their hand at spotting their future husbands. One belief is that if a girl stands naked over a lake at precisely midnight she will be able to see her future husband. Some of the folk magic must work as Juhannus is the most popular weekend to get married.

For those in Helsinki who cannot escape to a summer cottage, Seurasaari is the preferred place to celebrate Juhannus. Just over 10,000 Finns gather to see the large bonfire at the parkland island, which is preceded by the Midsummer wedding. A happy couple are wed in the 17th Century Karuna church then dance a bridal waltz. But rather than drive off on honeymoon they row a boat out to set a torch to the 10m-high bonfire that blazes into the night. Though as midsummer is the height of Finland’s midnight sun the night never goes much past twilight. This makes for a long night of drinking and a hangover that takes a public holiday to overcome.

Correction: This article has been updated to reflect that Friday's Midsummer eve is a public holiday which also features bonfires. Additionally, a reference to kauppatori was removed, due to confusion with Helsinki''s main market square, Kauppatori.

The article 'Finland’s midsummer madness' was published in partnership with Lonely Planet.