We confess that we believe in "one, holy, catholic, and apostolic church." Catholicity meaning, of course, that the church is universal, that the church is not regional, not provincial, not limited to one ethnic, racial, or cultural group.
In America during this season, we're in the midst of a season in which the catholicity of the church is sometimes not clearly confessed. The reason for that problem is the occasional in-church celebrations of Memorial Day, Independence Day and other such national celebrations.
Such things don't belong in the church. Several problems spring to mind. First is that the church is an embassage of heaven, and as such, is not specific to any country where a particular Mass is celebrated. Faithfulness to the liturgy precludes such nationalism; there's no place in the Mass for it. Related to this is that the Mass is God's, and secondarily that of all God's people, and no one should be excluded. If the American flag pledge is recited in the context of the Mass, someone who is not an American cannot participate. (Leaving aside questions of whether it should be recited at all).
Bottom line: national celebrations are offenses against the catholicity of the church. They should not be part of the Mass. Having them prior to or immediately after the Mass also gives the obvious appearance that they are a part of the Mass.
How to deal with national celebrations? A couple of suggestions:
1. We are commanded (I Timothy 2.2) to pray for "kings and all in authority." So do it. Liturgical prayers for those in authority should be in every Mass. I'd also argue -- on the catholic principle -- that prayers should be included for rulers of other nations who are known to be dealing with serious issues.
2. Prayers should be in the Mass for those occasions when there are serious problems going on for one's nation.
3. American flags (and other flags, including the so-called "Christian" flag) have no place in the church.
4. Faithful, ongoing teaching should be done involving a right understanding of submission to governmental authorities.
5. It goes without saying that governmental officials should never speak in the church in their official capacity, which includes campaign appearances. It's the nature of politicians to use any forum possible for campaigning, but the church should have nothing to do with campaigns. If a politician shows up for the Mass, that's wonderful, but should be given no recognition that would not be given to any other person.