Depends on what you’re doing and how it affects how you dress.
I’d agree with 20 F if you’re heading to work or a party or something where you 1) don’t want to show up all sweaty, and 2) have to wear decent looking clothes.
If I’m going out for a snowshoe or ski, it can get much colder than that - down to zero or below is fine, if you can dress for it. Meaning, many layers of clothing that is difficult to take on and off. And once you get moving, you’ll probably be sweaty.
Snowshoeing, for example: you have to dress warm enough so when you start out or pause for a bit, you won’t immediately get cold. So that means that while you’re moving or in the sun, you are slightly too hot, so you sweat. On a really cold day, that usually means long underwear (tops & bottoms), another layer on top (a sweater and jeans or snowpants), then a shell or jacket of some sort, parka if it’s really cold or if I mean to be out a long time. Then thick gloves, hat, neck warmer. Two layers of socks. A faceguard or scarf to pull up over my face at times.
Obviously, if you’re walking to work, you can’t wear all that, because 1) none of it is work-appropriate clothing and 2) you’ll be a sweaty mess when you take it all off. But it’ll keep you warm in very chilly weather.