February only has 28 days to allow for the other months to have 29 or 31 days, according to the rules set by the Roman king Numa Pompilius. The original Roman calendar had 10 months and began with March. After the reorganization, February was used for purification rituals.
The Roman calendar consisted of 304 days, with an unaccounted for 61 days in winter. Historians believe that this is because the Romans primarily used the calendar for planting and harvest, which was done in the winter.
The calendar reorganized by Numa had 29 or 31 days for the first 11 months because even numbers were considered unlucky. February was left with 28 days. When Caesar added 10 days to the calendar, he kept February with 28 days so the pagan rituals could continue.