Yes, while its down IS a good time to invest overall while things are artificially low, but right this minute its doing nosedive after nosedive. Wait for that to stabilize for at least a few days in a row.
Its okay in the current market to put in 1000 dollars and have it be 950 tomorrow, because it'll go back up in the near-ish future. But you don't want to put in 1000 dollars today and have it be 500 dollars tomorrow.
I don't want to sound rude here, but nobody knows. If you have that information that it will lose 50 % tomorrow, and doing that consistently, you would certainly be the richest man in the world.
Anyway, nobody knows. So it should be done with prudence, it could well lose more than 50 % tomorrow, or win 10 % or double. Who knows. It is impossible to time the market, and that is a game in which nobody should participate.
It doesn't matter a lot a little decline if you have a long-term investment horizon, the only indicator to know a little bit about this is the "Buffett indicator" or the CAPE ratio, but it is impossible to know what the market is going to do. That's why Bufett prefers to invest in individual companies, in which you can know whether it will be successful or not, and you can know whether the valuation is attractive or not.
How do you know if the valuation of an entire index is high or not? Perhaps Buffett indicator, but that could have worked better twenty years ago, not the interest rates are virtually zero, and that implies that valuation go to infinity.
The other way to know is if estimating the overall growth of the economy over long time periods. Let's say… Perhaps USA is going to grow only 2 % every year for the next 20 years, because it is way bigger than it was before and there is less room to grow and so on... And then you can assume what inflation will be... let's say 3 % over the next 20 years. So, S&P 500 return will be around 5 %, being conservative. If you expect it will grow more than that, it will be because of what people will pay more/less or because interest rates will be lower, which right now are at zero. So, you might want to take a look at the overall P/E of the S&P 500... And compare it to the interest rates right now, or to the historical, which is around 16. Right now, P/E is 18. Sometimes it is more, sometimes it is less, and it depends a lot in interest rates and in the overall emotional state of the market. And then add that additional return to the 5 %. But it is difficult to expect it to be way bigger than 20 consistently. It could be way lower too, but it is not that far away from the historical average.
Investing in an index is not riskless if it is done in a short-term basis, or even a 10-year basis, but it will be a lot better after 20 or 30 or 50 years. That is important. Also, I would prefer to invest in individual companies because then you have a better chance to earn more than 5 % or 7 %, when you invest in S&P 500 you have plenty of diversification, which could hurt your returns if you could instead pick the top performers rather than 500 companies. (You might also make a mistake and pick the worst performers).