As with any good trader or investor the best time to buy is usally when the market is stagnating or when others are selling.
I honestly believe if you have funds available December is a great time to buy players and invest into FI,let me explain why.
Players prices are dropping - This may sound really obvious but alot of players prices have dropped over the last 2 weeks and this is an opportunity to buy a player for cheaper than he has been for a while, months in some cases. December is always quiet but January is always booming so any players you buy now will almost certainly cost more come the end January. Opporutnities to buy proven div returners for cheap do not come round very often.
IPD - Buying a player now is perfect for IPD. Everyone knows the fixture list is brutal in England over the festive period so buying a player now gives nearly all prem based players a huge amount of IPD div opportunities. Wolves for example have 7 games in the next 30 days. Whether you buy for IPD typically or not you can not argue that Xmas is a busy time so buying players at reduced cost with the added extra games for IPD is a huge positive.
Div opportunities + winter break - As mentioned above, the xmas period is a very congested time for football teams. Lots of matches to be played, lots of gold and silver days so plenty of opporutnities to win divs.
So to conclude buying now means you are getting players at a cheap price but with instant chances to win dividens, i believe almost any player brought now will be worth more come start of Febuary but you would of also had the opportunity to win IPD and PB divs.