Thanks, GUEST (your link has a few pop-ups, but is otherwise very useful. Here's my notes for these two:

1100-1200 - TRUE. Hildegard of Bingen was 100 years ahead of her time, though, as the idea didn't catch on immediately. There's some more information here.

800-900 - FALSE. Asser does describe Alfred's celebration of Christmas but, unfortunately, no description of the mummers play surfaces for another 850 years or so. Alfred was defeated and nearly captured by the Danes under Guthrum while observing Christmas at Chippenham in 878.

Any ideas on who might have moved to Poland with their guitars?

:-)