Summer Solstice is 4 IBU, Winter Solstice is 6 IBU, that's straight from AVBC. They add their only hop addition during the whirlpool, I like to add mine during the last couple minutes of the boil (4-5), and leave them in while I chill the wort.
Summer Solstice is basically identical to Anchor Steam (California Common), but it has almost no IBUs, and they add vanilla. Winter Solstice is the same as Summer, just a bigger grain bill. If you hop the beer more, you'll basically have a Vanilla Cali Common.
I haven't had a Winter Solstice in awhile, so I'm not sure if it's as dry as the Summer solstice. It's possible the winter might need a higher mash temp, for a higher FG, and you'd need to step up the SG to get about 6.9 ABV. I don't know, I just brewed the reciepe I posted yeasterday, so here in about 7-8 weeks I'll report back on how it turned out.
I've brewed the Summer solstice recipe twice, and it's turned out fantastic both times. It's an absolute favorite of my friends and family. Recipe is the same as the Winter, I cut back the grain bill to hit 1.053 SG:
1# Crystal 40
13oz Crystal 80