Usually November/December however it depends on what the weather was like during the year some years the olives are good and some years they aren't depends on the place and of course the trees...Last year the weather was good for the olives and picking was great in some areas .. However the weather this year hasn't been so good in some areas and some people wont be picking this year.............
I think you have to precise your question, are you looking for business, travel or personal use.
Regarding the business you should start looking from now, however, travel it can be arranged at any time, as some travel agencies organise some lunches out, but if you are looking for personal use, that you can get it at any time.
I hope you got what you need, if you are not satisfied I can add more information.