There is no time that s not good for buying a coin. Before considering the time, the first thing to consider first is security, after which time will follow.
When it comes to the time to buy, i always check the nature of the project. For a genuine project prefer ICO stage and if i miss that, i would opt for exchange.

The number one reason, of course, is the dip provided that the coin that you intend to buy has a good platform and developing, buying in ICO is also recommended as long as the ICO has the potential and they offer a good bonus from their pre sale and ICO stage.

ICOs gives huge profit if the project succeeds/there is a great increase of their token's value. But it depends on the amount you invest.
Bounties also are good for it, and you don't need to put money but your efforts.
The best time to buy altcoin is when you see it with low value and believe it haves potential and will have a growth on the value.
ICO/bounty stage is better than buying from exchange, but on some times, tokens/coins haves ICO price or even less, available to be bought on exchanges.
About exchanges and bounty, if you likes to invest on already stable coins (Bitcoin, Ethereum, Cardano, NEO, EOS, Siacoin, etc) I recommend you CoinBundle. There are the 20 top coins to invest as bundles (instead of choosing which coins to use on your portfolio, you buy a pre-built portfolio), without fees in a easy investment platform. Try it and tell me what do you think

You can buy altcoins anytime. It all depends on your motive to buy that coins. Depending upon the market conditions and your investment , profit is decided and it's up to you that how much profit you want to earn.

Now is the best time to buy altcoin because the price is still cheap, to be able to get a good altcoin, of course you have to be good at analyzing the coins because not all coins are worth investing in.

In my opinion, the best moment to buy Altcoins is the time from the end of ICO and access to decentralized exchanges before their listing on large crypto exchanges. As a rule, during this period, alts depreciate greatly, possibly due to the massive sales of tokens by bounty-hunters. And after listing on such exchanges as Binance, their price usually rises several times.

Now is the best time to buy all the cryptocurrency you intend to buy and keep them for the harvest time which is bull season. Those that invest now will smile later in the nearest future because bull run will surely comes.

It is impossible to guess exactly what time is the best for buying coins. The market is constantly changing and we should be able to analyze the graph of bitcoin prices.

I see that cryptocurrency market moves in cycles and if you can  understand these cycles it is the key of profiting and managing risk. As Buffet said:" There are two major ways of investing and gaining profit from it. Firstly, Hold more of the things that rise and less of the things that fall. Secondly, try to have more risk exposure when markets rise and less when they fall." So if you want to win in this game try to expand your horizons and knowledge in this realm.                        
                           
                           
                           
                           

Buffet's recommendations are unclear, and I doubt they've ever helped somebody. You need the practice to get a stable income in this market. You'll see yourself the year will be really hard.