it was to be a time when meaning

This construction is sometimes used in historical presentation. It is often preceded by an introduction to the theme.

For example. “The fifteenth century dawned {introduction}. It {the 15th century} was to be a time ...”

Here, the reader or listener is invited to place their thoughts at the beginning of the fifteenth century, and to look forward into that century. Hence “It was (looking back from our present viewpoint) to be (looking forward from the viewpoint of the start of that century) a time when ...”