According to a popular story, about 800 AD, Irene, the Empress of the Roman Empire in Eastern Europe and West Asia, wanted to become more friendly with Charlemagne, the ruler of the Holy Roman Empire in Western Europe. By making an alliance, they could both be more powerful (They were even thinking of marrying each other). Irene wanted to give Charlemagne a really impressive present. So she sent him a great treasure - the linen tunic (or dress) that Mary was wearing when she gave birth to the baby Jesus.
Charles the Bald (painted during his lifetime)
We don't have any way of knowing whether this is really the dress that Mary was wearing, but it probably wasn't really sent by Irene. More likely, Charlemagne's grandson, Charles the Bald, bought the tunic somewhere. Nobody knows where it was before Charles the Bald.
Charles the Bald gave the dress to Chartres Cathedral around 850 AD for safekeeping, and it has been there ever since. Thousands of Christian people came every year on pilgrimage to Chartres to see Mary's dress and pray to Mary there, as they still do today.
Check out some linen in a clothing store or fabric shop
More about Chartres Cathedral
Cathedral: The Story of Its Construction, by David Macaulay (1981). Beautiful drawings and clear text explain exactly how medieval craftsmen built a cathedral, from foundation to the stained glass windows. Easy reading.