Say you have $7000 day 1. Option 1 is to invest it all immediately. Option 2 is to invest only a portion and just keep the rest in cash until you invest later.

In the long run, the invested side always does better than the cash side, interest rates on a savings account by definition never match the long run gains in the stock market. You get a premium for your money in the stock market because it can go negative in the short term.

There is a reason to do Option 2, if you're saving for a house or a car or something you probably don't want to risk the market going down right before you want to make that purchase. Or if you are very sensitive to losses and you would he anxious or devastated if you put the money in and saw the value drop.

But for retirement funds, you want to maximize long term gains so it makes the most sense to put it in Day 1.