First of all, we are not talking about your regular scheduled contributions to your 401(k), 402(b), 457(b) or IRAs. For those contributions either by every paycheck period or on your own schedule, you allocate the money to buy funds. You should continue to contribute to your retirement accounts without fail. You should never time the market with your retirement savings!
This is not what we are talking about here! We are talking about the extra cash from selling a property, receiving a cash gift from your parents, etc. When and how do you put the cash into the market?
You need to wait for an entry point to get into the market. The entry point would be either a recession or a correction. A recession is by definition two consecutive quarters of negative GDP and a correction is the broad equity market down 10% from its high. When we say the broad equity market, we usually refer to S&P 500 Index.
At this point, we don't see a recession on the horizon. S&P500 Index has reached an all-time high of 3,027 on July 26th! 10% pullback will be around 2,725 level. While you are waiting for an entry point, I would suggest you to leave your cash in 3month CD for now. You can also put them in US treasury, check the following link.
When the entry point level has reached, you should put the money into the market with a dollar cost average method which will help spread your risk around.