How to convert dayname to number?

The most efficient way is not to use DayName but this:

DateValue[{2012, 12, 24}, "ISOWeekDay"]

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Regarding Robert's comment

Well, that was a correct answer but not on the exact question. What if I don't have a date but just a weekday as e.g. Monday how do I now get the ISOWeekDay number. Without explicitly coding. Is there a a ready to use MMA function f with f[Monday] == 1

the shortest solution I found is

ISODay = System`DateObjectDump`$dowAssociation;

ISODay[Monday]
(* 1 *)