From that Italian mistranslation: Shakespeare in Lodi

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.

Whatever: Shakespeare is Coming Soon