There is no true mating season for cats - they tend to come in heat more during spring and summer, but cats are induced ovulators, so dont have to be in heat to get pregnant. We had 9 kittens over X Mas, then took some 7 week old ones in in Feb, and this week have picked up mum and 5 day old kittens - so there is no way of telling, it also depends on how mild the winter has been too.
Totally indoor cats can have litters year round - but seem to have stronger heat cycles from February to September.
Our barn cats only had kittens in the spring/summer (May-Sept) - we had a few litters and they were always born between that time - never in late fall/winter.