for trading, when the trend is going down, you may want to hold of you already have investments in it. if none wait for the trend to reach its lowest peak then once it starts to rise its the best time to invest your money  wait for it to go as high as possible then sell it. it is just like stock trading the only difference is there are couple of ways to obtain bitcoin and the trend of cryptos seem to be longer than stock market. as we know that the market is down as of today, we must wait before we let go of our current coins and wait for it to start rising before buying some more.