Not sure...but certainly, Riccarton and Riverton both raced at Easter. Riccarton on Saturday, Monday ( I think ) and the following Saturday. Riverton on Saturday, Monday and Tuesday. There were, in those days, sufficient horses/ riders for those days to clash without a problem. Often horses would come up for the last day at Riccarton and the Great Autumn Hcp. A worthwhile target for the Riverton Cup winner.
Riccarton changed all that by choosing to go away from the traditional Easter dates, which are dictated by the Church calendar. They felt that, as the dates were not consistent year on year, that connections could have more surety in programming their horses' racing.
I think that was the reason for the Ellerslie shift too.
Some may think that was/is a sensible choice, others may not.
Riverton retained it's traditional Easter focus. With the alarming drop in both horse and rider numbers, it's just as well...although at the time, I don't think that could have been envisaged .