Trading on low-volume days in the market could incur substantial slippage. People who make trades — especially large trades — on low-volume days often will not find many partners on the other side of the order book willing to make that trade. The result is that they could end up paying much more for the trade than expected, incurring slippage in the process.
Always pay attention to Bitcoin. Most altcoins (every cryptocurrency except Bitcoin) are pegged more closely to Bitcoin than Asian currencies were to the USD during the Asian Financial Crisis. If Bitcoin price pump drastically, altcoins price can go down as people try to exit altcoins to ride the BTC profits; inversely, if Bitcoin prices dump drastically, altcoin prices can go down, too, as people exit altcoins to exchange back into fiat. The best times for altcoin growth appear when Bitcoin shows organic growth or decline, or remains stagnant in price.