If you miss the first couple of snows, it’s fine to sow the seeds right on top of the snow (though they may not germinate quite as robustly as those sowed directly onto the ground). With a little sunshine, the darker seeds absorb and heat up melting themselves down into the snow. Better yet, the next snow buries the seeds down more deeply below the surface. Just as sowing prior to the first snows, seeds are moved into soil by freezing and thawing as the snow melts later.
I don’t recommend sowing on top of the snow if your yard gets a lot of wind. Wind can blow the top layers of the snow and seeds to another part of the landscape or your neighbor’s yard! In windy areas, it’s best to try and get underneath the snow earlier in November of December.
To make sure the perennial wildflower seeds are subjected to a long enough stretch of cold, moist conditions, try to get the seeds sown by February. Note that a mixture of annual and perennial wildflower seeds can be sown using this method. The annuals will sprout nicely even though they don’t need the damp winter cold.
Be patient and by late spring/early summer you should see lots of small seedlings establishing themselves into your landscape.