The best time to buy bitcoins is when a supposed 'crash' happens. A good example of this would be yesterday, when prices fell to $13k and it has risen again today to $15k. If you bought bitcoins yesterday, you would've made a very large immediate profit. I would say now is still a good time to buy as the prices are still recovering.

Also, if you buy at a peak and see prices fall, don't panic sell as this will result in a loss. If you do buy at a peak, just ignore short term price movements and keep holding onto you bitcoins.