I would suggest learning about the calendar in the order that I have it laid out here. First familiarize yourself with the calendar and each ring using Your Calendar and the Quick Ring Reference.
Next I have given information on Rings 23 – 25 and the Reconstruction of the Runic Calendar here
Ring 3: The Anglo-Saxon Lunar Months
Ring 4: The Gragorian Calendar
And Finally, The Runic Year Calculation