When is good time for buying bitcoins, that is individual, first someone need to have reason for buying something.
Here we come to holding, if the reason of buying is profit then best way is long term holding, for that price is still great, its under 800 $ and it will go on more then 1300 in next couple years, that means buying now is 450 $ + profit per one bitcoin in couple years. That sounds great doesn't it.