As a rule of thumb, it's better to buy when the market's in the red than when it's doing well. As Warren Buffet said, "keep buying it through thick and thin, and especially through thin." (He was referring to an S&P 500 index fund.)

In a bull market you get things for cheap, with the expectation that the bull market will end long enough before your retirement that those assets will increase in value. You basically bought those assets at a discount. Bonus!

Closer to retirement, you wouldn't want to take on that much risk, so you'd move (over time, as you get closer to retirement) to more stable investments. Target-date mutual funds[0] do this automatically.

[0] https://en.wikipedia.org/wiki/Target_date_fund

You always do it so you don’t have to question the timing. If you do it during a bill, you’re waiting until the price is going up. Doing it during a bear gets you when the price is going down. Buy low sell high and all that jazz.