According to this article:
there is evidence to support investing on October 28th. Some years it works out, some years it doesn’t. But if you average it out, it’s probably the best way to go.
I often have clients do the monthly purchase for convenience and for something called ‘Dollar Cost Averaging’ during which a regular purchase buys units at whatever the price happens to be that day – averaging it out over a period of time often means you’ll have paid less per unit than the average cost over that same period of time.
But a lot of people also have lump sums they like to do just in time to get the tax break. If this is what you are doing, it can be argued that March 1st is not a great time to invest and maybe a little forethought could result in a greater rate of return.
Just because you have a deadline, doesn’t mean you should aim for it!