Who decided what day it was?
This is perhaps the most interesting question. In an age where everyone (at least within a given country) adheres to the same timekeeping regime, its not obvious what are the advantages of a uniform time system as we just take it for granted.
Perhaps an obvious answer might be the Druids. They were supposed to be revered by everyone nationally and they could be in a position to provide consistent information about the timing of festivals. They might have needed to control access to stone circles to keep a monopoly on this knowledge and also meet up between themselves to decide on which particular days festivals should take place. It might look odd if they did not synchronise the events and undermine their position if adjacent settlements celebrated these events on different days.
The Druids might also have dictated when it was a good time to sow particular crops or forage for nuts, fungi, etc. As long as their prediction were right most of the time, they might have got away with it.
The problem I have with the Druids being in control of these things and dictating the right days/times for major events nationally is that many settlements were remote from each other and communications were often poor, especially in winter. It's much more likely that these things were decided by the local 'great and good', perhaps with more or less involvement with the local Druids depending on circumstances. It would have made for interesting discussions I would have thought. A chapter in Year of the Celt: Beltane deals with just this issue.