The best time to buy Bitcoin is every day. At least if you only have fiat to loose. Selling Bitcoin makes no sense, unless if you didn't understand the concept.

Bitcoin is a much better, more honest and more sustainable form of money than fiat. I stopped considering Euros or Dollars as a reliable measurement of value quite some time ago. These currencies will be inflated to death, that's a certainty. With Bitcoin however, you will actually improve your wealth, because even if the amount you hold stays the same, you will be able to buy more with it in the future. Its deflationary function is a blessing.