There is no Easter without Christmas; there is no point to Christmas if there is no Easter.
Mary did, in fact, know.
Knowing how God is, He had the stars ready at creation to move into position exactly for the the wise men to see when Jesus came.
You can research and discuss when and where Jesus was born, but while you do that remember the point of the birth doesn’t change. God came down into the dark with us so we could have Light. God knew we couldn’t pay for our sins on our own, so because He loves us so much He did it for us.