Easter is linked to the Jewish Passover festival. The Jewish calendar is based on the moon.
Since AD325, Easter Day is defined as the first Sunday after the first full moon which falls on or after the spring equinox. Tables to calculate this date have been in existence for many centuries. Sunday, of course, because that was the day of the week Christ rose from the dead.
The observance of Christmas was not part of the Christian religion until the fourth century. Early Christians adapted the existing midwinter celebrations (in the case of the Romans, Saturnalia, or the birth of Mithras - a Roman god) for this purpose, and 25 December chosen as a suitable date. Despite many attempts by scientists and biblical scholars, the date of Christ's birth is not known.
It was not thought important to link Christmas Day to any particular day of the week.