weekday represents a day of the week in the civil calendar
.  It normally holds values in the range 
0 to 
6,
corresponding to Sunday through Saturday, but
it may hold non-negative values outside this range
.  It can be constructed with any 
unsigned value,
which will be subsequently truncated to fit into 
weekday's unspecified internal storage
.  weekday meets the 
Cpp17EqualityComparable (Table 
28) requirements
.  [
Note 1: 
weekday is not
Cpp17LessThanComparable
because there is no universal consensus on which day is the first day of the week
.  weekday's arithmetic operations treat the days of the week as a circular range,
with no beginning and no end
.  — 
end note]