In astronomical terms, the longest day of the year takes places on either 20 June or 21 June.
With the Christianisation of Latvia, Jāņi was moved to the day before St. The day before Jāņi is called 'Herbal day' (Zāļu diena), when people will gather up the flowers and plants that will be used to make bouquets and wreaths to be worn on Jāņi.
In Northern Europe, its effects are more pronounced with very long days, which of course is contrasted six months later when the winter solstice (Ziemassvētki) results in very long nights in that part of the world.Midsummer’s Eve, Swedish Midsommarafton and Midsommar, Finnish Juhannus, Danish Sankt Hans Aften, Norwegian Sankhansaften, holiday celebrating the longest day of the year in the Northern Hemisphere, the summer solstice (June 21). During this time many Scandinavians travel to rural parts of the country. In Sweden the holiday is officially observed on a Friday between June 19th and 25th, whereas in Finland it is officially celebrated on a Saturday between June 20th and 26th, though festivities begin the preceding Friday evening.Although an ancient feast, midsummer is still an important national festival in Finland, as well as in Sweden and Norway.Midsummer is celebrated on June 24th, but in Finland (in year 1955) and Sweden (in year 1953), the date was moved to fall on the first Saturday after June 19th, on the initiative of labour organizations.