This should make it all clear ;-)

http://www.cursuswalker.co.uk/metonic/

That webpage sums up the concepts of the lunar and solar measuring pretty well. Its measurements must have been crucial for the farmer/settlers in the past, if we also consider some/many megalithic monuments (callanish, recumbents, etc) to be alligned on the moon. The sun is fairly easy to measure from a fixed position during the year, and to mark its movements easy. The harder part was the moon.

In terms of calendars, the pre-christian calendar may have been a 13 month long year, ie one with lunar months of 28 days each, with one day extra every year. That extra day may have been the one devoted to New Year.

Leap years are to do with keeping the "proper names and dates" of each month in the right place.

Every four years each day would slip back one day.

So in eight years you'd be two days out and so on.

So every 100 years (bear with me, I'm doing this in my head) the calendar will slip 25 days or so (call it a month, so every month would be a month out of wack (?)

So, in 600 years Christmas would be in Summer (?)

THAT SAID
If prehistoric people essentially want to know when to plant and harvest crops, the actual day is unimportant. March 3rd could infact now be December the 5. The date given to the day is not important. Its position in relation to a lunar and solar cycle is much more relevant.

Much of our calendar is to do with the working out where Easter falls as it is a moveable feast unlike say Christmas Day. although that also is based on lunar events and counting days (not including Sundays I believe), to correctly celebrate Lent, Good Friday, Palm Sunday, etc.