Hey,
Me and my friends have just booked to go to Zante in July
We personally booked 'early' because we didn't want to take the risk of not being able to find anywhere nearer the time but I guess that's up to you however if you do some research you should be able to find some good January sales (we managed to save over £200 per person) but like I said its really up to you if you want to take the risk or not.
I would personally recommend booking with a travel agent just because it is convenient i.e you don't have to worry about booking flights, transport to and from the airport as well as having to wait for your room to be ready when you arrive etc. They also usually come with a rep as well, not that I would advise buying any deals off them as they will be cheaper off club reps on the strip but they are useful for telling you where the best places to go are.
As for tickets to events literally just google Zante 2016 events or something along those lines and something should come up; but you could wait to book them when you get to Zante.. But if you want to go on the Rum & Raybans booze cruise I would book it soon (or whenever you book your holiday) because apparently it sells out and you can save some money by booking early and just think of all the alcohol you could buy with that extra £10-20
Hope that helps!