How would we know that, bitcoin will never goes down more?
We wouldn't thats why its called playing the market
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)
You can sometimes predict it by checking the graphs. If it went down a lot in the past days and is below the average monthly price it's a good idea to buy (especially if the price just started to slowly go up after a drop).