Depends very much on what you are growing and what space/facilities you have available. I sow seeds every month of the year apart from July/August...although I have just made a start on the hardy annuals this week, as I want them to germinate and make sturdy little plantlets to overwinter.

Sorry, I should have been clearer. I'm referring to sowing in place.

Sowing what? This is an absolutely perfect time for direct sowing of hardy annuals such as centaurea, ammi, nigella, larkspur and quite a few fast growing vegetables such as leafy greens, radish, lettuce, spring onions, as well as garlic bulbs. Also, hardy perennials can go into the ground as they will not germinate without a suitable period of cold, so will simply bide their time until spring. The only risky plants are the fast germinators which are not hardy enough to overwinter as small plantlets...such as osteospermum, lavender, pelargoniums, salvia. What had you in mind to sow?

Hardy perennials - primrose, coreopsis, columbine, to name a few.

Yep, I am just about to sow a bunch of stuff, including primroses and aquilegias. I only sow in place if I have lots of seed...but if you have been saving your own, then you probably have plenty. I have recently sown digitalis, patrinia, agastache in my greenhouse and have direct sown ammi.. There is also a lot of self-seeding going on - early summer perennials such as dianthus, campanulas and such, All in all, there is a lot to do and if possible, I will keep my options open with a mix of different sowing strategies and timings. No reason not to get started asap, since seed will be ripening and falling anytime from now until late autumn.