As U.S. markets climb to new highs and the Dow Jones industrial average comes within points of the 20,000 mark, investors may be making two mistakes if they are waiting for a decline before buying into equities.

Jeremy Siegel, professor of finance at the Wharton School of Business and author of the classic "Stocks for the Long Run," said in a recent interview that when investors anticipate a significant drop — say, 20 percent — they typically do not consider the fact that the stock market may in fact rise between current levels and a large decline.

"First of all, it goes up 30 percent in the meantime, so even then they'd be better off" just sticking with their holdings, he said Tuesday on CNBC's "Trading Nation."

On top of this, they may discount the fear they'll feel once the hoped-for decline actually transpires.

"Once it drops 20 percent, these guys are never going back in," Siegel said. "You know, 'Oh my god, look, the market's falling apart, I'm not going back in.'"

Instead of waiting for a correction or a crash, investors are better off just buying now, he said.

"I see the earnings recession, I think, is over. We're seeing a meaningful acceleration. Interest rates, yes, are going up, but still very low. Stocks still have huge edges over bonds, and other investments, in my opinion," he said.

"I think stocks represent really good long-term values now," Siegel concluded.