As long as you invest your spare money, you can buy it anytime. Bitcoin is the perfect speculative product. Sooner or later, it will rise. It's just a matter of time. Since it is idle money, put some in the wallet and remember the private key.
Please don't get fool by buying "anytime" because it has an advantage when you bought at a cheaper price compared to buying at a high price. This a thing we need to consider before buying and even it was just spare money for you, you can't just simply put it at risk and lose it. Of course, our goal to earn money, not to lose them and that is why we should take the best opportunity, otherwise.