You would be wrong on all counts.
1. The Sabbath was the covenant ceremony which looked forward to Christ. Exodus
31:16
Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations,
for a perpetual covenant.
When the covenant ended (Heb 8:13) so did the Sabbath observance. The celebration of Sunday as the day of the glorious and life-giving Resurrection replaces it. You celebrate a day which denies that Christ has come and obtained salvation for us. We celebrate the Resurrection on the day when it happened.
2. The earliest Christians used the earmarks in the Bible to come up with the 25th of December as the date of Christ's birth. They based it upon the time of Zacharias service in the Temple and Elizabeth's conception and birth, and then the conception of Mary and the visitation of Elizabeth to her.
Should you observe Sunday? I dunno. What does the Resurrection of Christ mean to you?