Possessive form

One would say a two day, not a two days. Otherwise, i don't see what the question is.

a hyphen is optional --

A two-day medical certificate

AS has been already said,

A two day medical certificate
A two-day medical certificate

or

A medical certificate for two days.

By the way, this is nothing to do with the "possessive form".

Unless "2-days' "

I ask myself "In what sense do the two days possess the certificate?". I am aware of such forms as "two days' growth of beard", which is a different thing - the growth of beard of two days.

Con Darned if I know, I'm no linguist. If the certificate requires two days of data to compile then I guess the completion is 2 days' work in much the same way as birth is 9 mos' labour

OK except a two day medical certificate is a certificate, issued by a doctor, saying that the patient should take two days off work, starting on the date of issue. The patient gives it to his employer or a state benefits office e.g. to get sick pay.

I am inclined to think this should work like phrases such as
(sometimes hyphenated)

ten ton load
hundred mile journey
three hour wait
two day absence
ten gallon hat

that is, number, countable thing - singular form, noun

I have to agree it's a tricky one but nonetheless I can see those 100 miles as possessing his journey or 2 days as possessing a fellow's absence

It's a bit tougher with the load and the hat

Hi all,
thanks. I thought I must write two day's MC like a day's leave.

What are you trying to say, Jane, what's your intended meaning? Perhaps you meant, "A two day medical certificate has been given to me".