Sunday in Hebrew is "yom rishon", or "the first day", since the days of the week in Hebrew are named according to the order of creation. Therefore, Monday is called "the second day", Tuesday "the third day", and so forth.
Sunday was the day that The Creator created light, the first of the creations. The Zohar (Jewish esoteric commentary on the Torah, based on the teachings of Rabbi Shimon Bar Yochai from 1900 years ago) explains that the first creation corresponds to the first commandment of the ten commandments, namely, "I am the Lord your G-d". As a result, the more a person develops spiritual cognizance and crowns Hashem as his/her G-d, then the more that person illuminates his/her own soul.
Unhappiness and negative emotions are directly proportionate to the level of darkness in one's soul. Conversely, the more a soul receives divine light, the more one feels happiness and inner peace. When we strengthen our faith, we illuminate our souls - it's that simple.
Have a wonderful week. Warmest regards to all my friends in Boulder, Colorado, and a special thanks to you, Leah.