Rabbinical Remarriage

My posts being more Jewish this week, shortly after my wife died, a Jewish neighbour told me that they have a law: their rabbi, if his wife dies, must remarry within a month. She said: "It's a very old law. It must have had an origin that made a lot of sense." OBSERVATION: Supposing that it takes the rabbi a week to get a grip on himself after the shock, then a week for the first woman to turn him down, another week for the second woman to make up her mind ... It would be interesting to know how it actually works ... My own wife, before she died, instructed me to marry E., to whom I am now married. But that was more than two years before it all came together.