“Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice, or that take place on the 24th of June and the preceding evening.
European midsummer-related holidays, traditions, and celebrations are pre-Christian in origin and have been superficially Christianised as celebrations of the Nativity of St. John the Baptist as “Saint John’s Eve” festivals. They are particularly important in Northern Europe – Denmark, Estonia, Finland, Latvia, Lithuania, Norway and Sweden – but are found also in Ireland, parts of Britain (Cornwall especially), France, Italy, Malta, Portugal, Spain, other parts of Europe, and elsewhere – such as Canada, the United States, Puerto Rico, and even in the Southern Hemisphere (Brazil), where this imported European celebration would be more appropriately called Midwinter.
Midsummer is also sometimes referred to by neo-pagans and others as Litha, stemming from Bede’s De temporum ratione in which he gave the Anglo-Saxon names for the months roughly corresponding to June and July as “se Ærra Liþa” and “se Æfterra Liþa” (the “early Litha month” and the “later Litha month”) with an intercalary month of “Liþa” appearing after se Æfterra Liþa on leap years. The fire festival or Lith- Summer solstice is a tradition for many pagans.
Solstitial celebrations still centre upon 24 June, which is no longer the longest day of the year. The difference between the Julian calendar year (365.2500 days) and the tropical year (365.2422 days) moved the day associated with the actual astronomical solstice forward approximately three days every four centuries, until Pope Gregory XIII changed the calendar bringing the solstice to around 21 June. In the Gregorian calendar, the solstice does shift, but in the long term it moves only about one day in 3000 years.”