Well, pretty much.
New Year’s Day – sure it’s a day, and the start of the new year – according to our calendar. But the Roman year started March 1st, the Chinese New Year is in February, etc. Lots of different dates to start a new year. But it’s a common holiday in many cultures.
Martin Luther King Day (and Washington’s Birthday & Lincoln’s Birthday aka President’s Day & Columbus Day etc.) – those were real people, and the dates chosen are generally fairly close to their actual birthdates, so they are not wholly made up. But the celebration is local to the USA – other countries have other heroes that they celebrate.
Passover, Easter, etc. – most cultures have some kind of springtime/planting season celebration. Often linked to their predominant religion. Whether this is based on any reality depends on how much faith you have in Jewish, Christian, etc. religious stories.
Mothers Day (and Fathers Day, Grandparents Day, Secretaries Day, etc.) – basically completely made up & assigned to a particular day to honor a group of people. Or, if you’re cynical, to sell lots of greeting cards, candy, flowers, etc.
Memorial Day – well, obviously, lots of soldiers have died in various wars, so this isn’t completely made up. But the choice of this specific day is pretty arbitrary, and varies in different countries.
Independence Day – specific to USA, based on a real incident in history (but it probably happened on July 2nd, not July 4th). Most countries have some such patriotic holiday, but when in the year varies. But it usually is based on some historic date.
Labor Day – another day just chosen to celebrate working people. So basically made up; no specific reason for the date. In USA it’s at the end of summer; many European countries have it at the start of summer, May 1st.
Veterans Day – well, it’s related to a specific historical incident – used to be called Armistice Day. Many countries have some such celebration, on various days.
Halloween – originally a religious eve celebration. Assignment to this date is pretty arbitrary. Whether it’s made up depends on your belief in Christian religion. But for most people, they don’t even remember the connection to religion – it’s just a holiday.
Thanksgiving – typical fall/harvest celebration, as is common in most agricultural cultures. Supposedly related to a specific semi-historical incident, but may be largely legend.
Christmas, Saturnalia, etc. – typical solstice holiday. Roman one was credited to their god Saturn; Christian version is credited to birth of Christ (though historically that probably occurred in March), etc. Again, whether it’s ‘completely made up’ depends on how much you believe in Saturn, Christ, etc.
So it seems that most cultures have some holiday celebrations. But the specific dates & names given to them are pretty much entirely made up.