When the down trend is finished and a new up trend is forming.
But don't buy too early - wait for the trend to turn and the chart to show confirmation that the down trend is finished.
I like to use count back line analysis as a buy signal = buy on the when price closes above the height of the third lowest significant candle = green line. But not before.
This charts suggests to me that we will see lower lows before higher highs.
A solid base has yet to form.