Sort of.

Christianity kept many holy days that had been celebrated by other religions, but not necesarily because they were trying to sugar coat their new faith and make conversion easier.

When you look at early Christianity's spread, particularly in the countryside, you see many areas becoming Christian, but continuing to do what they always did before conversion. This has been interpreted through a letter written by Pope Gregory the Great around 600, in which he urged a missionary to borrow temples and holy days in order to convince people to accept the new faith.

But we can't use that story to explain what happened earlier, because a lot had changed by 600. In particular, the church had a lot more social muscle than in the early days.

It's become aparent to historians now that Christians and 'pagans' (which I put in scare quotes because there wasn't a single pagan church, the term was Christian jargon for 'everyone else') lived together theough most of Roman empire, not in separate communities. And most Christians were comfortable participating in 'pagan' relgious celebrations like public festivals, theater games (both which had religious overtones, like most of Roman public life - there was no separation of church and state in the Roman empire), and even probably sacrificing to the Roman emperor (see Eric Rebillard's brilliant little book. Christians and their Many Identities). This was especially true of Roman elites who converted: they didn't change all aspects of their life when they joined the new faith.

This meant that converts brought a lot of practices with them when they entered Christianity, some of which we would consider 'pagan,' but which most late antique Christians seem to have been ok with (except for loud mouths like Tertulian, but it seems clear that few listened to him when he said Christians should look and act differently from the rest if the Roman world). Roman elites, who had - before conversion - used their money to build and repair twmples, started building churches. Not because they were changing their practices, but because they were staying the same. They just kept doing what rich romans did (temple building), only directed toward the worship of a new god (this caused a lot of tension with bishops, who wanted to be in conteol of the new churches, but didn't have the social influence to stop elites from making their own worship centers until well into the fifth century; see Kim Bowes, Private Worship, Public Values). Ordinary people kept doing what they always did, except replacing the names of local gods with the names of Christian saints. Not because their bishops were trying to ease them into the new faith, but because that was how ordinary people worshipped, and they didn't see a need to change their methods, so long as they were worshipping the proper god (or saint).

It's like science in the modern world. Christians and atheists both base their lives on it, even though science challeneges many aspects of religion. But everyone, except a few radicals on the fringes, trusts science because it's how the world works. In the ancient world, holy days were the same way. Dec 21 was important, as anyone who uunderstands the movement of the sun will recognize. Christians knew the day was holy just like their pagan neighbors did - when they decided Jesus' birthday was on the 25th, the fact that this date was during other major religious festivals confirmed their conviction that Jesus was the son of god. Because that was a good day for gods to be born. They shared the date because they agreed it was meaningful to everyone, not because they were sneaky pragmatists who were willing to invent stuff in order to rob pagans. That came hundreds of years later, after the church had cemented its control of Rome and started to encounter new groups of 'pagans' as it expanded outward.