Final answer:
February is the only month that consistently has 28 days. The lunar month is approximately 29.5 days, and systems like the Metonic cycle have been used in lunar calendars to keep them in sync with the solar year.
Step-by-step explanation:
The month that consists of only 28 days is February, except in a leap year when it has 29 days. The lunar month, or the time it takes the moon to cycle from full phase to full phase, is approximately 29.5 days, which inspired early versions of the calendar. To keep such a calendar aligned with the solar year, a system akin to the leap year in the Gregorian calendar was used, known as the Metonic cycle. This system proposed adding seven extra months over the course of 19 years to harmonize the lunar calendar with the seasons. The Ethiopian Coptic Church employs a related system, with 12 months of 30 days and a 13th month of 5 or 6 days to balance the calendar year.