For the short-term gain, wait for the dip, buy then, wait for the price to bounce back - sell. For the long term holding, buy anytime, or better yet - do a cost averaging, buy a little regularly (i.e. every month). Nowadays would be the best time. Thr market value of Bitcoin is currently falling due to thebseason but we don't need to worry about it becsuse it is expected that its market price will increase and become more expensive after holidays.