The observance of Easter and Christmas is not a pagan practice. It is celebrating historical events in the life of our Savior. The cycle of the Christian church year which includes Chistmas, Epipahny, Lent, Maundy Thursday, Good Friday, Easter, the Ascension, Pentecost, etc., is a yearly walk through the birth, life, death, resurrection and ascension of our Savior. The major holidays like Christmas, Good Friday, Easter, and the Ascension are precious to those who understand the significance of those events for our salvation and life as Christians.
Of course Christmas and Easter superceded pagan festivals. These festivals occured at the time of the winter solstice and the vernal equinox. Christian celebrations superceding pagan celebrations shows pastoral concern for those who were new in the faith.
There is no command in the Bible to celebrate Christmas or Easter on any special day. Likewise there is no command in the Bible to worship on Sunday or Saturday or Friday or any other day of the week (lets not get into that here). Christians in the freedom which God has given them have chosen to worship on these days.

Grace is not common.