The tradition started years before I had children of my own.
Number recognition and comprehension was an oft-cited IEP goal for my resource students, and holiday celebrations were a bright spot in our school year; somehow, I got the idea to make a hundred-day countdown to Christmas. Our classroom countdown consisted of a number line, hand-drawn and measured out on several sentence strips, posted above our door and down the hall. Each day we would step out and attach a construction-paper holly leaf and berry over the next largest number. The kindergarten teachers from around the corner would pause on their way to lunch each day and have their students read the last number showing, too.
I quit teaching when I got pregnant with our firstborn. Our preemie was quite verbal by the age of two, and by three was asking the requisite number of questions that preschoolers must ask every day (somewhere in the hundreds, I think). When the timing of Christmas came up, I decided to resurrect the countdown in the form of a paper chain, with a loop taken off each night.
"How many days until Christmas, Mommy?"
"Is the chain short or long, honey?"
"Then it's a long time until Christmas."
After a few years of making and remaking the paper chain, I got the bright idea to use pipe cleaners...and our chain got more elaborate. The days until Halloween are orange and black circles; Halloween to Thanksgiving, brown and gold; Thanksgiving to Christmas, red and green. Sometimes the last ring is a fancy mix of the two colors. The chain is hung over the children's bedrooms from pushpins that never come down, a parade of colorful loops taking up that side of the hallway. It must be adjusted every ten days or so, as the dangling end gets too high to remove a loop.
Our children are grown now, one in Japan, the other in college four hours up the road. But every September 16th, I'm compelled to make the chain, if only for my own countdown needs. I take a loop off each night, mentally noting the length remaining, marking the time until we celebrate as a family once again.