S&P500 index: buy signal

I use a simple system to tell me whether it is time to buy or sell the S&P 500 index (this index is a good proxy for the stock markets in general):

1) First filter: I go long only if the price is above the 200 day moving average (MA), i.e. the blue line on the chart. And I only go short if the price is below the 200 day MA.

2) Second filter: If the 20 day MA (pink line on the chart) crosses the 50 day MA (yellow line on the chart) to the upside, it is a buy signal. To the opposite, if the 20 day MA crosses the 50 day MA to the downside, it is time to sell.
This system worked pretty well in the past as you can see on the chart above

Yesterday the S&P500 index moved above the 200 day MA: first condition met. And two days ago, the 20 day MA crossed the 50 day MA to the upside: 2nd condition met. Buy signal was therefore activated.