Sunday, Easter, Halloween and Christmas are pagan holidays that Christians should never take part in, if we are truly followers of Christ and obedient to God and His word. If we are going to keep holidays, we should keep the holy days God established and all His people, including Jesus and His disciples and the early church apostles kept from the days of Moses (and Eden in the case of the seventh day Sabbath) until they got pushed aside in favor of the pagan ones — God’s moedim, the holy convocations and feasts that He appointed and clearly laid out, including the dates and how they are to be kept, in Leviticus 23. This is an excellent article on God’s feasts https://www.psalm11918.org/Articles/The-Moedim/#page-1 .
We have been given the great gift of God’s Holy Word, and if we are truly His children, we have been given the gift of His Holy Spirit of Truth to guide us into all truth. We have to put aside all the false teachings of men that we have been fed all our lives, and look to God’s Word alone for truth because His word is truth. “Sanctify them through thy truth: thy word is truth.” John 17:17. Our sanctification before the soon coming Day of the Lord and our being found by Him, fully evidencing our faith in, our love for and our devotion to Him who redeemed us, following Him and keeping all the commandments of God, should be of significantly greater importance to us than celebrating a pagan holiday.
“Here is the patience (endurance/perseverance) of the saints; they that keep the commandments of God, and the faith of Jesus.” Revelation 14:12.