In the period between Passover and Shavuot there is a commandment to count the Omer. In the time of the temple there was a sacrifice brought the day after Passover called the “Omer” and the Torah commands us to count 49 days from the day the Omer is brought and consecrate the fiftieth day as the holiday of Shavuos, the day of the receiving the Torah.
The reason for this commandment is to prepare us to once again spiritually receive the Torah anew. The Kabbalah teaches us that a person has seven basic emotions and each emotion also contains within it the remaining emotions, for example at times the emotion of love and kindness calls for severity and judgment for the benefit of the person you love. We therefore count 49 days, each day elevating and refining one element of our emotions and this prepares us to once again receive the Torah on Shavuos.
The verse in the Torah states that you should start this counting period from “the day after the day of rest.” The sages of the Talmud based on the explanation of the verse passed down from Moses interpreted this day of rest as referring to the first day of Passover and thus the counting should begin from the day after Passover regardless of which day of the week that might be. However the Boethusians, (a heretical Jewish group who lived towards the end of the Second Temple era) took this phrase literally and insisted that counting the omer should always begin on a Sunday. In order to force the Temple service to follow their interpretation of this verse, they even went so far as to try to rig the Rabbinical court’s declaration of the new month so Passover would occur on a Sabbath.
Why was it so important to begin the counting the day after Passover? The commentaries explain that changing one’s emotions is a most difficult task. The famed Mussar Master, Rabbi Yisroel of Salant is quoted to have said that it is easier to study the entire Talmud than to change even one habit. We therefore need to begin this process the day after Passover, a day when G-d took us out of the physical and spiritual limitations of Egypt, this day thus gives us the energy to also go beyond our own spiritual limitations and refine our emotions.