In Kathryn Warner's book on Richard II I found that in 1386 the usual meeting of the Order of the Garter was deferred from St George's Day, although no reason was given in the King's letters. This threw me as the opening scene of This New Spring of Time is that very meeting and I had written it as taking place on St George's Day. Suddenly, this was wrong!
So I searched as one does, and found that the Garter statutes (albeit compiled in Henry V's time) provided that if the feast of St George fell close to Easter, the meeting would be deferred until the following Sunday. This was to save the knights having to travel over Easter, which was a no-no.
In 1386, St George's Day fell on Easter Monday! So at a stroke, I knew why it had been deferred and the likely replacement date. Simples!