Perhaps you are thinking of Eph 5:11

"Do not participate in the unfruitful deeds of darkness, but instead even expose them"

This scripture certainly is taught, and should not be ignored simply because something is popular. However, Christians will always have different opinions on some topics. We don't agree with Christian participation in Halloween, but some other Christians add Christmas and even Easter to that category. These are issues that we should be convinced in our own minds about how God wants us to live, but the Apostle Paul warns against judging the motive of others:

Col 2:16-17
Therefore no one is to act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day --  things which are a mere shadow of what is to come; but the substance belongs to Christ.

He also wrote about being careful not to alienate himself from those outside of Christ to whom he brought the Gospel:

1 Cor 9:20-22
To the Jews I became as a Jew, so that I might win Jews; to those who are under the Law, as under the Law though not being myself under the Law, so that I might win those who are under the Law;

21 to those who are without law, as without law, though not being without the law of God but under the law of Christ, so that I might win those who are without law.

22 To the weak I became weak, that I might win the weak; I have become all things to all men, so that I may by all means save some.

In a sense, the answer to your first question about who has the right to "Christianize" a pagan holiday, is that God is able to use whatever means He desires to reach the lost. We need to be sensitive to what He does and does not want us to include or exclude from our lives.

1 Cor 10:23
All things are lawful, but not all things are profitable. All things are lawful, but not all things edify.