People are available for jams after the fruit harvest ;-)
Joke aside, they are available when they have free time like during the summer holidays and when the weather is bad (too cold or too hot) and forces people to stay indoors.
This makes a lot of sense, but how would you accurately time a jam though in these periods as it seems that the giants have already hogged the majority of the community during these times, they have pre planned positioning of their jams and they seem to be featured automatically wiping the effort of the smaller jam host before they get a chance as it is.