We don't know who the speaker is or what it means to "have a mind of winter" or why somebody "must" have it. Frankly, it doesn't even sound all that fun.
But we soldier on anyways. After reading one line, our best guess is that one has "a mind of winter" like someone has girls or boys on the brain. The wintriness invades every thought that the mind makes. It has a wintry disposition.
All we know so far is that such a mind is necessary for… something. We'll have to keep reading to find out what.
To regard the frost and the boughs Of the pine-trees crusted with snow;
Apparently, once someone has a "mind of winter," they can see the winter scene: frost, boughs (a.k.a. tree branches), snow, etc.
But wait a second. You don't need any special winter brain to see those things. Shmoop likes to think we have a mind of summer, but we've seen our fair share of snow and frost.
So what gives? Maybe the key is in that word, "regard." If regarding something means seeing it in a specific way, looking at it closely, with careful attention, well then we imagine a mind of winter might come in handy.
And have been cold a long time To behold the junipers shagged with ice,
Lines 4-5 are set up a whole lot like lines 1-3. The gist here is that you have to have been cold a long time (which sounds a bit like having a mind of winter if you ask Shmoop) to see frosty junipers.
So being really, really cold makes us a better seer of nature in wintertime? Sounds dubious. But again, maybe there's something to be said about the diction here.
After all, behold doesn't just mean see. When you behold something, you see something impressive, awe-inspiring, even miraculous. As in, "Behold! A tap-dancing lobster!"
So when you behold, you're seeing like nobody's business.
We're getting the sense that when you have a mind of winter, and you've been cold for a long time, it let's you see all that snowy, slushy mush around you in a new and exciting way. You see things once more! with feeling!