It's a nice story, Mark, but I have my doubts. Einhard is writing a generation later and he was writing to glorify Charlemagne. It's a nice touch, showing Charlemagne's supposed humility, but implausible. Scholars have noticed that if you were trying to get someone canonized and they didn't do anything else in their lives, you could at least make the point that they were humble. I give this one about about as much credence as the story about Charlemagne's ancestor St. Arnoul of Metz who turned back a raging fire by making the sign of the cross and multiplied the beer so that it served everyone ;)
There are two parts to my statement. The first is that the pope probably consulted astrologers. The second is that astrologers helped choose the date. You might be reading those to mean that the pope made his decisions based only on astrology, but that's not what I said and not what I meant.
The political situation was complex, as it is always is at any time and place. The pope had his struggles with the Eastern empire, which had claimed since the fall of Rome to be the heir also of the Western empire. In choosing to make Charlemagne the new Western emperor, the pope was taking a radically new direction. Charlemagne was (arguably) the most powerful ruler over whom the pope had nominal authority, so he was a good candidate, but that misses the point that the pope wasn't forced to take the step at all.
As far as choosing the date, medieval people didn't just get on airplanes and spend a few hours in a city. Charlemagne's trip to Rome was a lengthy journey and he spent some time in the city. Not only that, he had been in Italy several times before, on military campaigns.
Significantly, Einhard tells us that it was Alcuin, Charlemagne's chief adviser and the man who taught him astrology, who urged Charlemagne to make this trip. Charlemagne traveled in November 800 and was in Rome already on December 1, when he held a council there. He wasn't crowned until some 3 weeks later, on Christmas day.
I would suppose Charlemagne heard mass every day, like other medieval kings. I would find it incredible if he traveled to Rome, but heard mass only privately in his own quarters until, just that once, he decided to visit a Roman church.
I also find it hard to imagine that Pope Leo III, who was one of the popes who actively protected astrologers, would not have used them himself. The popes who banned astrology did so because they thought it was demonic and a denial of free will, not because they thought it was wrong. And, anyone savvy enough to be a pope would also be savvy enough to know what their enemies' astrologers might be saying.
What I would imagine is something like this. The pope hit on the plan of designating Charlemagne as emperor in the West, which would be flouting the claims of Empress Irene and could be disastrous. The obvious choice would be Charlemagne, who already controlled most of the old empire, and certainly the parts the pope cared about. Astrologers would be brought in to consult. "Yes, certainly, the chart we have for Charlemagne shows he would be an excellent choice. Right character, chart shows high ability as soldier and politician, and nothing here to suggest his impending downfall."
Then they would have turned to horary astrology, very popular in the middle ages but rare nowadays. The pope would have asked "Should I make Charlemagne the Western emperor?". And the astrologers would have drawn a chart for the time the question was asked. Here we can cuss a bit, because this chart would have been the decision pivot, but we have no way re-creating it because we don't know when the question was officially posed (unless the chart has been preserved in some corner of the Vatican archives). Presumably, the chart showed good things. Of course, we should remember that a cagey astrologer might have had some influence on setting the time, so some limited control over what the answer would be. It's the kind of question that can be made more positive or more negative by just waiting a few hours or days before officially asking.
Then pope would have turned to mundane astrology for choosing a date. It would be hard to beat the high solemnity of doing it at Christmas mass. but run that past the astrologers too, just to make sure those two malefics Mars and Saturn won't be in the wrong aspects on that day. I imagine the astrologers probably looked at several dates. I would guess the December 1 conference was one of them, and Christmas mass was another. It's possible that the astrologers said "Oooo, go with December 1" but the pope liked the symbolism and marketing opportunities for Christmas better. Just because Pope Leo would have consulted astrologers wouldn't mean that he always listened to them ;)
As it happens, Charlemagne's coronation chart is among the most famous in astrological history because it is considered to be the foundation chart for Western Civilization. (Similarly, the signing of the Declaration of Independence is the "birth chart" for the United States.)
I'm a little skeptical of the chart because I can't figure out where the information about the time of day came from, but if true it has Aries rising and the Sun conjunct the Midheaven in Capricorn. That's very nice timing, but not too hard to achieve for a December date with just a little bit of fussing about the exact time. It's the chart an astrologer would have chosen if asked to make sure Charlemagne's subsequent career was glorious.
I don't think it's possible to raise any serious doubts about the role of astrology in political decisions of this time.