In 1622, the papacy declared January 1st to be the start of the New Year for all Catholic countries. Some protestant countries (such as England) refused to follow this rule, with the latter only switching to January 1st as the beginning of the year in 1750.
How common was it during the Middle Ages to start the year on March 25th (Annunciation Day)?
For example, some sources from the 15th century, such as this version of Ruralia commoda by Pietro De Crescenzi, and this book of hours (Les Très Riches Heures du Duc de Berry) indicate January as the first month of the year.
There are different and somewhat conflicting ways of starting a calendar in the Middle Ages, most notably:
Anno Incarnationis/Annunciationis: The year starts on March 25th.
Anno a Nativitatis Domini: The year starts on December 25th.
Anno Circuncisionis Domini: The year starts on January 1st.
As it is well known, the territory of Spain had a number of kingdoms in the Middle Ages, and each one used a different calendaric system, that of course changed over time.
-Catalan counties, later Principality of Catalonia. The dating system was based on the reigns of the Frankish kings. In the synod of Tarragona of the year 1180, it was decided, and soon applied, the usage of the Christian Era, and the system of Anno Incarnationis.
-Kingdom of Aragon, and kingdoms of Valencia and Mallorca. The dating system was the Anno Incarnationis until the year 1350, when king Peter IV changed it to Anno a Nativitatis Domini. In Valencia and Mallorca the change was applied in 1358.
-Kingdoms of Castile. The ordinary dating system was Anno Incarnationis, and the year was Era Hispanica (starting on 38 BCE). King John I substituted this with the Christian Era and the calendaric style of Anno a Nativitatis.
-Kingdom of Navarre. They used Anno Incarnationis from the year 1234 onwards until the XV century, when it was changed with the Anno a Nativitatis.