You can buy if you want now, but i would advise you to wait for bitcoin to fall even harder, because you can make more money that way. I think that bitcoin will fall very hard and that one of the altcoins will rise up and take over the place of bitcoin. So you can also invest in some altcoins. Lite coin for an example is the sleeping giant.
The best time to buy is when it price dump or fall. Set a value and then when it reaches it grab the opportunity to buy and set again for the second time when you want to sell your bitcoins. Bitcoin price change every seconds you just need to wait for the dump and pump of it. It was already proven that when the bitcoin price dump the next to it was pump.