On the dips and when the capital is available, of course.
Many dudes here would advise to go for buying when the price is really low. Its good. But as the price has increased and being stable now, buying would be a secondry option and saving the btc would the prior option. Still people can buy the btc now as we predict that the price may still go up. The more we save, the more benefit we get.
When the price is suffering a slow movement, most traders want this opportunity to grab some coins since once it goes for a rally again, they have the advantage and benefits.
In general, it's all about your guts to face the risk after you buy coins in that given price.
It is simple when to buy bitcoins, just always remember the traders move. But at low price and sell it on high price.
Don't wait for the price to go up before you are going to buy because it can be worst thing to happen if you are going to buy bitcoins today.