What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
Why The Sun is not solid, and its visible surface rotates roughly once every 27 days as seen from Earth. An active region of sunspots or a coronal hole that faces us this week rotates out of view and then comes back around about four weeks later, so the ionizing UV and X-ray flux, and therefore HF conditions, tend to repeat on that 26 to 28 day rhythm.
Watch out The Moon's orbit is close to the same length (about 27 to 29 days), which makes it tempting, but the Moon has no meaningful effect on ionization of the ionosphere; it is the solar flux that drives HF propagation.
27 days = one solar rotation. Same sunspot group, same conditions, four weeks later.