The best time to buy altcoins is when bitcoin price is rising. the logic here is that, most of the time, altcoin price will fall when bitcoin price rises. In that case, you can buy more altcoins when the prices are low, and that is when bitcoin price is high. Now that bitcoin price is high, it is the best time to buy altcoins and hold them for long time. You should only trade or sell these altcoins when the bitcoin price starts to fall down or when the altcoin price rises up.