Best time to buy altcoins is depends on different different situations. First if you are able to hold long than you can enter market with best altcoins as market is in deep. Second thing is to follow the top 200 alts listed on CMC on their social media sites like twitter and medium to get latest news and update from their projects. Also the websites like Kryptokal & coinmarketcal to get lates updates and events of the cryptoworlds. This industry runs on news and trends collect those trends and use them as whenever there is news appear about testnet, Mainnet, Forks, Node integration and partnership their price increases.