First, do not buy altcoins (not even ltc) if you are not sure about why you are doing it.
About buying btc I think it's always the best time, but be aware that on 10th December futures trading will start, and you will see either a big pump or a big dump
Futures trading won't start in a big dump (of the price). These futures contracts don't trade on the blockchain so they won't influence Bitcoin's volatility. However,
any firm selling futures contracts on the US market will need to own Bitcoin to cover the reserve requirements for all of their trades.