I'm not aware of any reference either, which is why I'm skeptical of this particular argument. You ask a good question. Would the average Roman know of the solstice or care? I'm not sure.
And if December 21 was not a great religious celebration, what were the dates of the great religious celebrations of ancient Roman? Surely one of those great celebrations would have been a better date to celebrate Christmas than some obscure date with no celebration at all.
I'd think so, yes. Here is a Roman civil calendar dating from 354; I don't know of an earlier one. http://www.tertullian.org/fathers/chronography_of_354_06_calendar.htm
Interestingly enough, the summer solstice is mentioned, but not the winter one.
This is very interesting. I enjoy the history.
Most of these speculations about the origin of celebrating Christmas start with the premise that the Church needed to appropriate a pagan holiday for its own. I think that premise is wrong. Rather, the church in her wisdom choose to honor the birth of the savior of the world. What ever reasons she had for choosing December 25 are secondary and perhaps in the end irrelevant. Merry Christmas every body.