How do you discover under-rated about to explode crypto-currencies such as ethereum ?
You research them. Ethereum grew so fast because it's a "platform" for other cryptocurrencies and technologies. There are others like it: Bitshares, Waves, Lisk, Stratis, NEO, etc. Look on coinmarketcap.com for the most popular and/or undervalued cryptocurrencies and start researching them.
> Are there any specific times of the day/week you buy/sell?
It doesn't work like that. You will have to learn trading technical analysis, but you can start by following some Youtubers that talk about altcoins and why they think coins are undervalued on a given day/week. Tradingview.com is also nice for technical analysis on cryptocurrencies (follow the top authors).