“How dare you celebrate Janus the two-headed god!”
There is a significant difference between using a calendar and elevating a pagan ritual to the status of being used, approved and encouraged in a Christian Church.
There is no difference. During January I don't worship Janus and during Ash Wednesday I don't worship Odin.
The claim that this or that Christian ritual is based on pagan roots is often used to try to discredit Christianity. It's often based on faulty logic and often contradictory. If you have a theological problem with Ash Wednesday, that's one thing. Guilt by association is another.
We might as well get rid of Christmas (and a host of other holy days) because of pagan roots.
Now the adoration of Mary does have pagan roots and that is a problem, but that's because it is theological rather than ritual.