It is this dimension that makes it, in the words of the liturgy, “A day of joy and rest, quiet and security,” a day of holiness, a day when one acquires a neshamah yeteirah, an “extra soul.”
To a certain extent, the meaning of Shabbat (and this is what distinguishes it from imitations adopted by other peoples) lies in the fact that it is not a day of gloom, hedged in by strict prohibitions.
On the contrary, festivity is of the essence.
Even one who is newly bereaved or has a fresh memory of some other personal catastrophe must stop mourning when Shabbat arrives.
The neshamah yeteirah each Jew is said to acquire on Shabbat is really an augmented ability to rejoice in tranquility, to accept life with a feeling of wholeness and contentment.
Shabbat is a time to disengage oneself from workaday affairs—even reading, speaking, and thinking about them are forbidden.
Even when it comes to spiritual matters, vexation and anxious self-analysis should be avoided. The holiness of the day must be sought in a spirit of pleasure, relaxation, and ease.
–Rabbi Adin Steinsaltz
from The Miracle of the Seventh Day: A Guide to the Spiritual Meaning, Significance, and Weekly Practice of the Jewish Sabbath by Rabbi Adin Steinsaltz, p.4