According to the Catholic Encyclopaedia on the web, the idea of the Assumption of the Virgin derives from the Christians of Egypt and Arabia, and was Celtic before it was Roman - the Gallican church was celebrating it a few decades before Rome.

But all these early churches placed it in January at first. It started moving to August in the 6th century. Surprisingly, the doctrine of the Assumption only became Catholic dogma in 1950.

Like McGrath, I'd find it very hard to believe that anybody in Europe could have been unaware of the doctrine for most of the last 1500 years.