Often when thinking of sowing vegetable seed, the information closest to hand comes from the back of a seed packet. Regarding sowing dates, be wary of accepting this advice!
Firstly, the dates given are all possible ones and are not the best ones. Secondly, bear in mind that a seed supplier is happy for you to be sowing seed at the more marginal times when crops risk failing… whereas very little seed is needed when it is sown well and at the best date (as long as seed is viable, that is another issue).
Some examples of outdoor sowing dates include parsnips in late March, runner beans and kale in late May or early June, and oriental leaves, including rocket, in August. The latter respond to shortening days by making more leaves, unlike in spring when they tend to flower quite quickly. Also in autumn there are less flea beetles to make holes in their leaves.
I advise you to spend a little time checking the best sowing dates (my book Organic Gardening has dates for all vegetables), to save yourself seed and time, and for better crops.