The Feast of the Dedication of Jerusalem, not mentioned in the Old Testament, was part of first century Jewish worship, and was something observed by Jesus (John 10:22). The Regulative Principles of Worship might well have defined this observance as being “according to the imaginations and devices of men.” Yet Jesus had no problem with it.
For the Puritans, and also for the Baptists who authored the 1689 London Baptist Confession, it was a statement that they would not be bound by extrabiblical observances imposed by the state church. Today if a church member doesn’t approve of a particular activity, he or she is free to not participate. Or to find another church.
“There is a great difference between instituting some exercise of piety, which believers may use with a free conscience, or may abstain from if they think the observance not to be useful, and enacting a law which brings the conscience into bondage.” - John Calvin
Regarding national flags, or for example a special program honoring veterans, I refer to Romans 13:7 - “Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honor to whom honor.”