To determine the best time to buy a coin, you can refer to fundamental and technical analysis. It is also recommended to spend time studying the markets and trading strategies.
XLM can be a good investment as Stellar works with a lot of companies (IBM, Samsung Mobile, Smartlands, Stripe, HTC Exodus etc.). This confirms that the project is developing, which means that there is a possibility that the coin will grow in price. In any case, do not forget about the risks of working with cryptocurrencies: invest only after you have weighed all the pros and cons.