When do you think will be a best time to buy bitcoin and when is the best time to keep bitcoin first before selling?
The best strategy for buying BTC is to BTC-cost average, dollar-cost average, or value-cost average into it.
BTC-cost average: buy equal amounts of BTC at regular intervals (e.g. 0.05BTC per week), regardless of BTC nominal price
dollar-cost average: buy equal dollar amounts of BTC at regular intervals (e.g. $20 per week), regardless of BTC nominal price
value-cost average: buy more BTC when it is near the bottom of its historical range, less when it is at the top of its historical range.