This is a right time to buy bitcoin but never a good time to sell. We may even borrow to buy so that when next the price goes up it will generate more profit. Also do not used all you have to buy at once because the price may still be going down day by day. It may work fine if you divide your money into different batch and buy bitcoin by batch weekly within this period of time.
Buying and selling your bitcoin depends upon the needs of daily life. I mean that if you do not need money for your day to day life then hold your bitcoin in your wallet and wait for the price to grow more, and if you need extra money than your regular income then selling is mandatory, because respectful life is far better than money. Then there is no problem to sell out your bitcoin.