The earliest day that Easter can be is 22 March---an event that happens only once every 200 years. This year Easter is 23 March, something that comes once a century. This fluxuation is brought about by the ancient problems of determining Easter anyway. It combines a lunar calendar such as the Jews used, and a solar calendar such as the Romans used. The purpose is to keep Easter a) on a Sunday, and b) in the spring. In the history of the Church there have been other attempts to fix the date of Easter. Some wanted it to coincide with Passover, but that meant that "The Lord's Day" would not always be Sunday. The Church found this unacceptable since it had already been celebrating Sunday as the Lord's Day from the time of the Apostles. To celebrate the annual observance of his resurrection on a day other than Sunday would make no sense.
Thus, we have at the Council of Nicea (325) the formulation of the calculating of the date of Easter: The First Sunday after the Full Moon, after the Vernal Equinox. This has kept Easter in the spring, and on a Sunday, and more or less around the time of the Jewish Passover.
Between Ash Wednesday and now we are in ordinary time. This is so-called because the Sundays are ordered from one to thirty-four. This cycle is disrupted by Lent and Easter, but takes up again after Pentecost. The Sundays between Epiphany and Lent were formerly called Sunday's after Epiphany. They were followed by the pre-Lenten cycle of Septuagesima, Sexagesima, and Quinquagesima Sundays before Ash Wednesday. The reform of the liturgical calendar after Vatican II did away both with the Sundays after Epiphany and the pre-Lenten Sundays. This year we will have four Sundays of Ordinary Time before Lent.