The keyword to create ordinal numbers in a date has changed in MkIV.26. August 2015 19:36
In case that you were looking for an automated way to print the date,
here's one option:
\setuplanguage[en][date={weekday,{, },day:ord,{~},month,{,~},year}]
(And the documentation is somewhat wrong, I think it suggests to use
"day+" which doesn't work. But I should check the details again.)
You can’t show them as superscript when you use the \date commandUnfortunately I didn't manage to figure out how to do the superscript
automatically. Yes, it's 1\highordinalstr{st} like Alan mentioned, but
that only works in "manual mode".