Why can't I justify the indefinite article by referring it to the head word (holiday) which is modified by a descriptive attribute (seven days')?
I didn't take a seven days' holiday, but I took a ten days' holiday.
Or is there anything wrong with the indefinite article in the basic expression "to take a holiday"?
"ten days'" doesn't seem to be an adjective to apply "A/An+Adj.+N" pattern.
Why not think of it as an example of the Possessive Case - the way it's usually taught in Russia. Consider:
My daughter's teacher is very kind. - "my" clearly refers to "daughter", not "teacher"
My daughters' teacher is very kind. - the number of daughters I have taught by the same kind teacher is beside the point, still "my" as a determiner refers to "daughters", not "teacher".
If you are preparing your pupils for the Russian State Exam or similar, they could benefit from knowing that the Possessive Case can be used to show time duration as in your example:
"ten days' holiday" where "days" do not require other determiner than "ten".
Should I also mention English adjectives do not have plural forms,
as in your '"ten days'"? You can use "ten-day" instead.
I see no point in stating that I am a teacher, rather the opposite - I've come here to learn more. Unfortunately, no profile option can imply both.