When I have enough money & see a good price
This is when I am buying bitcoins too when I am seeing the price is acceptable and very low that I afford to buy then I will going to buy but there are times that even though the price is good but I don't have money in my pocket for buying so I feel sad about it that I am just bypassing the opportunity of buying bitcoins that time.
timing really is the best for us to gain profits from bitcoin industry if you place your buy orders in a low price in got fulfilled then expect for the rebounds and sure profits its up to you whether to do short term trade or hold for long for much bigger profits.