There’s at least a theoretical mathematical argument that the best way to get into the market—assuming you have a lump of cash sitting in a money market, bank account, or CD—is to go “all in” without waiting.
This is due to several factors. One is the “time value” of money. Another is the fact that the direction of the markets is impossible to predict—but on average, historically, stocks have gone up twice as much as they’ve gone down. Based on that alone, the argument goes, it might mathematically seem better to invest everything at once than to apportion your investment over time. And with interest rates so low, there’s a real cost to having cash sit idle for long periods of time as you invest it bit by bit. There’s also the camp that says, “If investing really is a ‘random walk down Wall Street,’ what difference does it make if you invest all at once or gradually over time?”