I was going to give you an answer - typing it all myself so that it looked like I actually knew the answer. Alas I have run out of energy and am off back to bed - once I turf one of the moggies off it. He will growl at me and then stomp down the bed and then, when I comfortable under the duvet, he will come and stomp all over me.
March the 25th is, and was, Lady Day, one of the four Quarter Days when ground rents were due. Quarter Days are still used in commercial leases. Being the one that marked Spring, when everything in a rural economy started, it was chosen as marking the end of one financial year and the start of the next.