Two mitzvot from the Torah constitute Shabbat: the positive mitzva of Zakhor (“commemorate”) and the negative mitzva of Shamor (“observe”). Shamor entails refraining from all melakha (creative labor), whereas Zakhor means remembering the fundamentals of faith. The first principle we remember on Shabbat is the creation of the world, and the second is the Exodus.
Although Zakhor is a positive time-bound mitzva, women are obligated in its fulfillment just as men are since Zakhor and Shamor were uttered together in the Ten Commandments and have equal status. Therefore, just as women are obligated in the negative mitzva of Shamor, so too they are obligated in Zakhor. Since men and women have an equal obligation, a woman can make kiddush (a fulfillment of Zakhor) on a man’s behalf. Yet a minor cannot make kiddush on an adult’s behalf, since minors capable of grasping the idea of Shabbat are only rabbinically obligated in kiddush, whereas adults have a Torah obligation (SA 271:2).
The Sages ordained fulfilling the mitzva of Zakhor by saying the Friday night kiddush over a cup of wine. There are two berakhot in the nighttime kiddush: Ha-gafen on the wine and Mekadesh Ha-Shabbat on the sanctity of Shabbat. The recitation of kiddush over wine immediately before the meal teaches that the sanctity of Shabbat is expressed on all levels – not just the spiritual aspects of life, but even the material aspects. It is therefore recited over wine, which brings joy, and before a festive meal.
The Sages further ordained that kiddush be recited over wine during the day, just before the morning meal. By beginning the meal with kiddush, it is apparent that this meal is significant and special, and it reminds us of the holiness of Shabbat. Since the basis of the obligation of the daytime kiddush is rabbinic, no special berakha was instituted in honor of Shabbat; only Ha-gafen is recited, though verses relating to Shabbat are customarily recited beforehand. The Shabbat-day kiddush is called “Kidusha Rabba” (the “great kiddush”) euphemistically, as the nighttime kiddush is more important (see Peninei Halakha: Shabbat 5:1-3 for more on these laws).
Like men, women are obligated to eat three meals on Shabbat and to have leĥem mishneh (SA 291:6l; MB and BHL ad loc.).