As a response to yesterday's post on the festivals connected to the barley growth cycle, Karli asked me a question about the timing of festivals. Since I thought others might be wondering about the same thing, I figured I'd copy/paste from the comments and answer her question here:

"I live in Pennsylvania, maybe an hour away from the Delaware River. As you can guess, the growing seasons and weather are very different than Greece's. In your Reconstructionist opinion, how do you feel about moving festivals to reflect barley's growing in my area?
 
Similarly, Rural Dionysia takes place close to our secular New Year and I believe was for celebrating the opening of the latest batch of wine. My grapes don't ripen until around September, and a wine made from them wouldn't be ready until March/April with only 6 months of aging (young for a wine, a year at least is better).
 
Is reflecting the reason for the festival more or less important than the ancient date?"

So, I am going to give you the TL;DR reply first: it depends on the festival. Is it an agricultural one that was tied to a specific event/occurance that takes place earlier or later in your area? Then change the date as you see fit. Was it a festival not tied to the agricultural cycle and/or untied to a specific event/occurance that takes place earlier or later in your area? then keep the original date in place.

The cycle of the Proerosia, Khloaia, Kalamaia, Plerosia, Antheia and Thalysia belongs to the former category. I suspect that these celebrations shifted year after year--save for, perhaps, the Proerosia--depending on the growth of the crops. The same was most likely true for the festivals related to the grape growth and harvest. That said, it seems as if the ancient Hellenes left themselves a bit more time for those--the Rural Dionysia celebrated the growth of the vine in general, while the Anthesteria, held on the eleventh to thirteenth of the month of Anthesterion (February/March), centered around the celebration of the maturing of the wine stored at the previous vintage. These can be addapted, yes, but you might not have to.

There are other festivals that are tied to the agricultural calendar but most likely do not need adjusting. These are almost all tied to the Eleusinian Mysteries and were pretty set in stone. They celebrate the agricultural as a whole and not a specific event. Some examples are the Thesmophoria, the Haloa, and for example the Skirophoria.

Then there is another element: are you celebrating it alone or with others? Set dates are important for a group of people, especially if they are not all in the same location. I suspect this even held true in ancient Hellas, as organizing festivities for a large portion of a large city like Athens was probably only doable if you knew year afer year when which festival was held. So even if your local situation does not reflect the event for the festival, it can still celebrate the spirit of it.

I can't speak for the Theoi, but I  suspect They are smart enough to figure out what's going on down here ith us. So in the end, do what makes you feel closest to the Theoi and your local growth cycle. If you celebrate with others, keep them in mind. All in all, we are reconstructing, not reenacting: it's not a show we are putting on, it is a religion we are practicing, and a way of life. So go with what feels right based on the information we have about the ways ancients. That is most likely the best advice I can give you.