I think it's a little odd that in the entire Names and Dates chapter (ND) there is no mention of the word "calendar", nor of the attribute @calendar. Despite the fact that att.datable is mentioned, and its @period attribute explained, @calendar is completely ignored:
Members of the att.datable class share the following attributes:
att.datable provides attributes for normalization of elements that contain dates, times, or datable events.
period supplies a pointer to some location defining a named period of time within which the datable item is understood to have occurred.
att.datable.w3c provides attributes for normalization of elements that contain datable events using the W3C datatypes.
when supplies the value of the date or time in a standard form, e.g. yyyy-mm-dd.
notBefore specifies the earliest possible date for the event in standard form, e.g. yyyy-mm-dd.
notAfter specifies the latest possible date for the event in standard form, e.g. yyyy-mm-dd.
from indicates the starting point of the period in standard form, e.g. yyyy-mm-dd.
to indicates the ending point of the period in standard form, e.g. yyyy-mm-dd.
Calendar description (<calendarDesc>) is rightly described in the header chapter (http://www.tei-c.org/release/doc/tei-p5-doc/en/html/HD.html#HD44), along with a single example of @calendar, but no other examples of @calendar appear outside the reference pages.
I recommend adding @calendar to the list of attributes included in the ND section quoted above, along with a brief paragraph explaining how to use it and including an example and a link to the HD <calendarDesc> section.