Each 24-hour cycle of day and night on Earth is known as a day. A day is the period of time it takes for the Earth to complete one full rotation about its axis.
There are 365.25 days in one year. The extra quarter of a day collects every four years to produce a leap year, which has 366 days. February is always the month that collects this extra day, meaning there are 29 days in February during a leap year. The leap day has been used far back as 46 B.C., when it was first added to the Julian calendar.