carnival's dates change every year, but they are not arbitrary - all catholic holidays are calculated based on the date of easter, including carnival.

carnival happens in exactly 47 days before easter sunday, which in turn is always the first full-moon sunday, calculated from march 21st.

march 21st is the day of the equinox, when day and night have equal length.

march 21st is also the day marking the changing of the seasons, and the first day of the astrological year.