The Calendar of Nippur
Temple of Sumer uses the calendar of Nippur mainly because the people of that city kept the best records. Modern archaeologists have found more about the months of Nippur than they have about the months of any other city, and they have done a good job doing it. Modern Sumerians and interested pagans generally like this calendar for these reasons.
Even if other cities had kept better records we would still have chosen the calendar of Nippur, because that city had another great thing going for them. The months of Nippur were stable from one era to the next. With all of the upheavals that happened throughout Sumer over the many centuries, the consistency of this calendar cut across all of them. We can be comfortable in the knowledge that Sumerians, at any time in their history would have recognized what the people of Nippur were doing.
The Calendar of Nippur gives a good religious counterpoint for modern Sumerians. Most modern Sumerians focus mainly on Enki and Inanna almost to the exclusion of other gods. Enki and Inanna were important, but Ninurta and Enlil were important as well. These were some of the gods of Nippur.
But do you personally have to use this? No. Obviously not. No two ancient city states had exactly the same holy calendar. Even countries in the modern era have wildly differing holidays and festivals. Other approaches taken by modern Sumerians are to take the calendar of Nippur and line the months up to the Gregorian calendar. Another approach is to take western holidays like Halloween and Christmas and celebrate them in a Sumerian way with Sumerian festivals.
The calendar seen below has been set up in a manner that can be used by modern practitioners. Suggestions for how the calendar can be used by Recons in northern climates where the summer and winter cycle is a little different than it is in the middle east are provided where they are appropriate.