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The convention of assuming the first of the year to be assigned to March 25 in 1155 (the ascension of Henry II) is due to Bond's "Handy-Book of Rules and Tables for verifying Dates". This convention is just an approximation. In England various conventions were used, sometimes simultaneously by different people.
In general, the church in England gradually settled on March 25, the Feast of the Annunciation, as being the first day of the year.