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day_of_week

Definition: atom_string dow = day_of_week(object y, integer m=0, d=0, bool bAsText=false)
Description: y: a year such as 2022, or a result from date() or a timedate.
m: a month 1..12, ignored/overidden if y is a sequence.
d: a day 1..31, ignored/overidden if y is a sequence.
bAsText: self explanatory, but should use a named parameter when y is a sequence.
Returns an integer from 1 to 7 (Mon..Sun)1, or the text version of it, ie
{"Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday"}[integer dow]
pwa/p2js: Supported.
Comments: Obviously this functionality is required as part of date(), and has been made available for general use.

if bAsText is true2, then y and m can be 0 and d 1..7, aka a date()[DT_DOW], for convenience.
Note however that as per the third line of the examples you still need to explicitly provide a 0 rather than completely omit y.

Not suitable for use with years prior to the introduction of the Gregorian calendar.
Returns 0 if y < 1752.
Example: 
?day_of_week(2015,1,1)  -- prints 4 (Thursday)
?day_of_week(2015,1,1,true) -- prints "Thursday"
?day_of_week(0,d:=4,bAsText:=true) -- prints "Thursday"
?day_of_week(date(),bAsText:=true) -- prints the current day

You may also, performance considerations aside, feel that 
if day_of_week(y,m,d,true)="Monday" then
    -- or format_timedate(td,"Dddd")="Monday", or
    -- day_of_week(0,0,td[DT_DOW],true)="Monday"

is clearer or otherwise makes debugging easier than the equivalent 
if day_of_week(y,m,d)=1 then
    -- (or td[DT_DOW]=1)
Implementation: See builtins\pdates.e (an autoinclude) for details of the actual implementation.
See Also: date, time, format_timedate
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