15 March 19

The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the Sun’s activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number and size of sunspots, flares, and other manifestations).

They have been observed (by changes in the Sun’s appearance and by changes seen on Earth, such as auroras) for centuries.

The changes on the Sun cause effects in space, in the atmosphere, and on Earth’s surface. While it is the dominant variable in solar activity, aperiodic fluctuations also occur. More at Wikipedia

The UCAR Center for Science has a lot of good articles on the sun….

The solar cycle is a 28-year cycle of the Julian calendar with respect to the week. It occurs because leap years occur every 4 years and there are 7 possible days to start a leap year, making a 28-year sequence. More at Wikipedia