The best moment to buy Bitcoin is that you must buy it for a low price. That is the best option that you have.
If you buy it for a low price, than you can sell it with a higher value and than you have a bigger chance to get some more money.
Can you please tell me which is best low to buy? or best high to sell? No one have answer of this as we cannot judge which is the best price to buy and which is the best price to sell.