Not a teacher.
The importance of these holidays will vary from person to person. Obviously, someone who is not a Christian will attach less significance to Christmas. Still, it is the major "end-of-year" celebration and occasion for giving and receiving gifts, so even those who are not celebrating it as the birth of the Savior, generally still find something to enjoy about that time of year.
Thanksgiving is probably the main annual occasion for family/friends to gather together. It was intended as a thanksgiving to God for a successful harvest, but is a national day and not a Church/religious holiday. The nice thing about Thanksgiving is that it is always on Thursday, so it means a 4 day weekend for most people. It features fine food, family, and copious amounts of (American) football for the men and shopping (the start of the Christmas shopping) for the ladies.
The 4th of July, Independence Day, is all about Summer. While it's cooking food on the grill, and fireworks and parades it is also the defining moment of our nation. The Declaration of Independence not only declared our nation's rightful place among the other states of the world, but it laid out a philosophy of governance that is a gift for all peoples who yearn to govern themselves without being under the rule of monarchs, tyrants and dictators.
Power does not come down from rulers appointed by God, but rather the people are sovereign and appoint representatives to serve the common good.
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. — That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness."