I suspect very few people think about how long a day is, and of that number, fewer still wonder if the length of a day changes much, or what sort of things affect the length of the day.
Fairly early, people realized that the days got longer and the nights got shorter as we moved from winter to summer, and the days got shorter and the nights got longer as we moved from summer to winter. Similarly, people realized early that the moon didn’t cycle its phases over exactly 28 days’ time[note]It’s actually just over 27 or 29 days’ time, depending on what you mean. See this article on wikipedia for more information[/note]. These changes are one reason why we don’t use a pure lunar calendar – lunar months do not cleanly match up with a solar year.
There are several “expected events” that alter the length of a day over the course of a year. Some have to do with the fact that the Earth does not orbit the Sun in a perfect circle. For example, the length of the day on the December Solstice is actually about 30 seconds longer than the average day, while the length of the day on the June Solstice is about 13 seconds longer. Conversely, the length of the day at the March Equinox is about 18 seconds shorter, and on the September Equinox it’s about 21 seconds shorter.
The length of a day will also change with tidal forces, and there are also effects due to the jet stream. Then there are “unexpected events”, like earthquakes.
Think about it – have you seen how an ice skater can change the speed of their spin by moving their arms closer or farther away from their bodies? If an earthquake “loosens” things up a bit, that has the effect of making the Earth slightly bigger, which means it slows down a bit.