The best time to buy altcoins is when bitcoin freezes in price movements. As long as bitcoin rises or falls, the altcoins markets tend to decline as money circulates between BTC and Fiat currency.
Well if you will observe the market, better to wait for the dumped market when you see almost all coins are falling including btc, the timing is really important, if you will be able to buy in a very low price and bitcoin start to rise up, the price of alts will also rise a little, the healthy market brings investors inside,expect the good green market and good profits after some months this scenario is just common inside crypto and more people already achieved to enjoy financial freedom just by making a good timing buying alts.