We know that holding Bitcoin for the long term has always been the best choice. The altcoin season is actually a good time for most of the altcoin holders to make profit and leverage their profit into accumulating more Bitcoin. I don't know how others are doing it but if they're not doing it then most likely they're just taking the cash and not going back to the market. Waiting for another opportunity to buy some of these alts and will wait if they pump again. The best time to sell Bitcoin is subjective to be honest.