As everything in gardening is dictated by the season then along with the regular jobs such as the watering, weeding and grass cutting, there are a variety of activities that come around only once or so each year such as the harvesting and preparations for winter. These are often easier to over-look till too late than the more frequent tasks.
the days are short, don’t waste a single dry day
Here I've listed the most important. Obviously the exact timing varies with locality, site and soil, and of course you may not grow exactly the same, but most plants have similar requirements at much the same times.
There are twelve months with, theoretically, four seasons. Each season can be divided into early, mid and late, and very roughly matched to a month for convenience. These matches are based on my experience here in East Anglia over many years, they will need to be modified for other places. Of course every year is different and the weather appears to be becoming more erratic still although the timing of tasks may change their order will remain much the same. Some tasks are noted more than once as they can be peformed in either of the adjacent periods, or they are just better repeated.
Early Winter is December, the days are short, don’t waste a single dry day