How do days work like it was just the 16th and now it's the 17th??

One rotation of the earth counts as a "day", a period of about 24 hours. One rotation of the earth around the sun is about a period of 365 days or a "year". A "year" is divided in a period of 12 "months" which are divided into a number of "days". We count the number of days as they progress and the time "midnight" (the ending of the last hour of the 24 hours of which a "day" is composed) is generally accepted as the point in time when the previous day becomes the next day so the 2nd day becomes the 3rd day, the 3rd day becomes the 4th and so on.

Better..?