Julius Caesar renamed July after himself, and August after Augustus Caesar. During the French Revolution, there was a new calendar made up to completely replace the old. Calendars change, and sometimes ego is involved. Plus, it may have been Obsid's idea...
I like this, although I wouldn't go with calling it Black Sun, if only to avoid Last Airbender references. Perhaps Shadow Sun, because of the dimming that occurs. I like Hollow Moon, but I feel like I've heard it before. I don't know what to do with that.The Winter and Summer Solstice festivals can stay, but they'd only be one day, or a weekend. Days of Black Sun (Total eclipse) and Hollow Moon (Total eclipse+Moxen pointed at Therinos), of course, would be (respectively) feared as momentary loss of connection to everything around and celebrated as an influx of the purest magic one can get without burning. The trail of the latter would be well-trodden with travelers, while the trail of the former would be shunned due to (for lack of a better term) heebie-jeebies. EDIT: Oh yeah, and momentary power drain and (in the worst cases) loss.
But I do like the loss of magical power during a solar eclipse. You could have it where two armies once fought, and the less magically dependent one attacked during a solar eclipse to win. The Lunar Eclipse, I feel, should be split into two events. In the first, the Lunar Eclipse, the moon's vanishing interferes with magic, increasing the likelihood of mishap. The Moxon + Eclipse would be the huge power boost. I'd actually say that it shouldn't be taboo to perform a ritual that channels this power, simply because there's very few spells that require such power, and none of them would be known by anyone other than the Minds. Such an eclipse would be the time when experimentation hits the high water mark, since the various universities would love to get ahold of a power boost even for one night.