The coldest I've ever ridden (commuted) in was 19 degrees F.
I have a ~7.25 mile ride and within 1 mile, I have a long uphill, so I warm up pretty quickly on that!
I use a HeadSweats skull cap and a Sugoi helmet cover and for really cold days, a neoprene face mask (newer ones are lined with fleece...I gotta get me one of those!). Not-so-cold days, I can go with just a balaclava pulled up over my chin/nose. It's easier to vent (i.e., shed) the balaclave when you need to breath, I find.
Hands require lobster gloves (3-fingered) and maybe a liner.
The rest I just layer, and try to make sure I leave room in the panniers to carry those layers in the afternoon if/when it warms up some!
But...I'm out of practice, having not commuted in some time. So all of this is from last year!