Stocks have been dropping lately due to the impact of coronavirus. When is the best time to enter SG stock market?

Never ever time the market. No one knows when the market will bottom out. If you already have an existing investment strategy, continue to stick to it. You could consider to buy more to average down if those investments are indeed good businesses.

If you have not started investment, you can consider doing a DCA on the POSB Invest Saver.

Totally agree with the rest. there is no best time. no one knows when is the bottom. those who predict correctly once will not be able to do it again.

which is why no fund managers can consistently outperform S&P500.

and also even Buffett can sometimes get it wrong.

so the main things are: diversify, and position size. but dont overdo it until you spend more time and money on screen than with your loved ones.