This is just a guess but...
In promoting Christianity to those with Pagan faiths, early Christian missionaries would plan for a Christian celebration at the time of popular Pagan rituals. At least, this is one theory why Christmas Day falls when it does - it is half way between the 'actual' day of Christ's birth and the (at one time) largest Pagan celebration of the year. Thus over a few years, the missionaries would change the belief system of a group by promoting their God as supreme, while being careful not to be offensive to the original beliefs. With the increasing influence of the Christian lifestyle, the original Pagan celebrations were all but forgotten. And we'll ignore the odd bit of bribery, torture and persecution of non-believers along the way. Naturally, the equinoxes and solstices would have been important days in a Pagan society more attuned to nature than we are today, and hence made likely targets for the early preachers of the Christian faith.
As to why two saints called John are involved ... I have no idea. I figure both these Johns are fairly important as far as saints go, and hence would have been matched to the larger Pagan festivals.
