I sowed some brassica seeds mid-January, and by mid-February the cabbages were up and sprouting their two cotyledons, or primary leaves. And today, about four-weeks post sowing, the first true leaves are starting into growth. As for the cauliflower seedlings, they have broken through the potting compost over about a tenth of the seed tray, perhaps only a day or two ago.
The following photo was taken about three-weeks post sowing, in mid-February. Unheated glass house bench, bright and well ventilated. I think this might be the tray of Golden Acre cabbages. It shows so clearly the two primary leaves fully emerged and greened-up.
And this afternoon we have an additional weeks' growth, with the true leaves beginning to emerge.