Holiday dates- I understand the feeling. And I'm going to ramble here so beware!
I feel that some of the holy days are pretty set... for example the Quarter Days (equinox and solstice) are pretty obvious and more or less un-affected by the actions of people. But the Cross-Quarter Days (seasonal festivals) celebrate things that were significant to a more agriculturally based society functioning in a very different age. I don't know if it's so much that the holidays are off, but that we no longer live in a community/culture/climate where X happens roughly halfway between Quarter Day A and Quarter Day B. Sure, the lambs still arrive in the spring... but that may or may not be the first week of February and so the traditional symbolism of Imbolc may not have any relevance to the lived experience of the modern pagan.
It's one of the many risks of shifting a culturally linked earth based religion to a different part of the earth (either geographical or temporal shift). A religion developes to meet the needs of it's adherents, and as those needs change the religion also changes. However, as a religion evolves there is the risk of certain elements becoming.... hmmmm... codified. Symbols become powerful or significant in their own right, not just because of what they signify.
For example, if you happen to have an ocean hard to your eastern hand, or a mountain blocking out the southern sky then it makes sense to honor water to the east, or earth to the south. And so you do, and your children do, and their children do. But then a great great grandchild moves to a place where the mountain is to the north and the ocean is to the west. Now what? The symbolic tie of east/water & south/earth is important to them. It's what they know as "right". It's what has always been "right". And it certainly isn't going to hurt anything to maintain that tradition and hand it down to yet more generations. But some day it's likely that a person might actually look outside their own door and say "hey, you know, I sort of feel like earth is a bit more of a "north" thing around here". And that's equally vaild and perhaps a better path for people there to travel since it is their actual physical experience of the elements.
Like the story about cutting the end off the roast. It's ok to maintain the traditions and make use of the energy paths carved by generation after generation. But at some point there will be something so obviously different that you'll have to say "I am choosing to maintain this tradition for it's own sake, not for it's absolute relevance to my own lived experience".
It's a difference you see between certain pagan paths... a member of a recon faith is going to be very focused on the "doing" of religion. There are rituals and there are requirements and these must be met in order to be valid. A god requires an offering of X. It is not ok to substitute an offering of Y because you happen to prefer Y or because you've never liked the way X smells. The correct action is to offer item X in the proper manner and for many recon traditions it's a question of right living and right action. It doesn't matter so much where "your" mountain is, you're following the path laid out over the generations. (and I say this with great respect as a one time follower of a recon path and as a sib to someone following such a path)
While in a different pagan path there may be much more fluidity... the difference between baking a cake and making a stew. When baking you can't just substitute baking soda for baking powder, a tsp for a tbls, add a little more milk instead of another egg, or bake at 250 instead of 350 and expect something decent to manifest in your oven. But a yummy stew is totally doable by eyeballing amounts and throwing in a bit of this and a bit of that and swapping flavors you prefer for ones you don't. So some pagan paths suggest taking a look around and finding "your" mountain and going from there.
Both are valid, but the two approaches attract different people and provide different flavors.
Oh, from wikipedia (cause it's the easiest to find), here's an interesting tidbit on the Irish calendar. It sort of explains why the holidas fall as they do (considering that many modern US pagan authors over the past 50 years have drawn heavily from early Irish seasonal cycles and festival dating).
"The Irish calendar does not observe the typical astronomical seasons (beginning, in the Northern Hemisphere, on the equinoxes and solstices), or the meteorological seasons (beginning on March 1, June 1, September 1 and December 1), but rather centres the seasons around the solstices and equinoxes (so that, for instance, midsummer falls on the summer solstice), beginning the seasons at the approximate halfway points between solstice and equinox, following the seasons of the ancient Celts (see below) which are pre-Christian in origin. This Celtic origin is particularly evident in the Irish naming of many of the months: some names, like May (Bealtaine), August (Lughnasadh/Lúnasa) and November (Samhain) were the names of pagan Celtic festivals. In addition, the names for September and October (Meán Fómhair and Deireadh Fómhair respectively) translate directly as "middle of autumn" and "end of autumn". Christianity has also left its mark on the Irish months: December is Nollaig, a word also meaning Christmastide."