The Earth’s side view does not tilt toward the Sun when it is Mehregan and Knowrooz. The light shines equally on Earth. That is why we Iranians have chosen the Spring Equinox as the beginning of our new year and also celebrate Thanksgiving in Autumn Equinox at 02:29 GMT about 7 hours from now.
In a civilization that spanned from North Africa to China, it was the only solution to have everyone agree that they were equal. No other calendar system comes close as it only loses one day every 15000 years.
All the other major calendars are meaningless. The Chinese is based on the New Moon in Aquarius. The Hindu is the New Moon in Scorpio. The Gregorian is 10th degree Capricorn. The Jewish one is a mix of Lunar and Solar. The Moslem is Lunar which loses 10 days every year. None of them have any celestial meaning.