ADF's official ceremonial calendar consists of eight high days spaced evenly throughout the year, which begins on November 1st of each year and runs through the 31st of the same year.¹ Each high day is approximately 6.5 weeks apart on average, with their times determined by the astrological phenomena of solstice and equinox.
Local congregations are given flexibility to celebrate the high days in a manner that is convenient to their schedules, so long as the time falls within a week. The Board of Directors has the ability to modify this set of dates on a local level, should a request for an exception be made.¹
Part of the reason ADF has chosen to set the calendar in place and require our Groves to celebrate eight feasts is because part of the definition of a church that the IRS uses is "regular religious services" and "distinct religious history." Within Neo-Paganism, there is a tradition of celebrating eight yearly high days on or about the dates listed in the ADF Constitution, and as ADF is seeking to be a public tradition of Neo-Paganism that is open to the (at least Pagan) public, celebrating high day rituals on these dates is most likely to serve the needs of the community at large. Also, utilizing these dates helps us to provide religious festivals that are based in historical festivals of certain IE cultures (even if they're not the festivals of all IE cultures) and develop a common practice.
Another key feature of the ceremonial calendar is that it does not tie a festival name to the date. In the original ADF Bylaws, the names of the festivals were Irish and tied to dates. The removal of the Irish names showed our committment to the other cultures within ADF to not force them into a box, and their replacement with astronomical phenomena allowed us to show that we were growing into a world-wide organization, since the southern hemisphere experiences different seasons, but the same phenomena.
¹ - The ADF Constitution includes the ADF Ceremonial Calendar in Article 4.
² - "Churches" defined, IRS website.