After twelve years of Catholic education, I still believe in what I was taught that Easter Sunday is the holiest day of the year. It completes the fulfillment of our salvation. Is that correct, still?
Actually, based on the reasoning you were given, Ascension would be the “holiest day of the year” because even the Resurrection was not complete until Christ ascended into heaven and opened heaven to us. However, Easter Sunday is the most important feast on the Christian liturgical calendar, and it is because it is the Christian feast to which all other Christian feasts either point or from which they take their meaning.