From the passage (Shmot 20:7) “Remember the day of Shabbat to sanctify it,” our Gemara learns that we are obligated to sanctify Shabbat by making kiddush not only at night, but during the day, as well. Rav Yehuda comments that the Shabbat morning kiddush consists solely of the blessing over wine – Borei pri ha-gafen.
The Gemara relates that Rav Ashi happened to come to the city of Mehoza. The Sages of Mehoza said to him on day: Will the Master recite for us the great kiddush? And they immediately brought him a cup of wine. Rav Ashi was unsure what they meant by the term great kiddush and wondered if the residents of Mehoza included other matters in their kiddush. He thought: What is this great kiddush to which they refer? He said to himself: Since with regard to all the blessings that require a cup of wine, one first recites the blessing: Who creates the fruit of the vine, I will start with that blessing. He recited: Who creates the fruit of the vine, and lengthened it to see if they were expecting an additional blessing. He saw a particular elder bending over his cup and drinking, and he realized that this was the end of the great kiddush. He read the following verse about himself: “The wise man, his eyes are in his head” (Kohelet 2:14), as he was alert enough to discern the expectations of the local residents.
One very straightforward question raised with regard to kiddush on Shabbat morning is why the simple blessing of Borei pri ha-gafen should be considered kiddush at all. It appears to be simply a berakha that is typically made over a cup of wine. The Mekhtam suggests that since drinking a cup of wine is a requirement specifically on Shabbat morning, it honors the Shabbat and, as such, is considered to be kiddush. The Tosafot Ri”d adds that during the week someone can choose to include wine in his meal or refrain from doing so. Since the cup of wine opens the meal on Shabbat, it is appropriate to begin with kiddush.
The expression Kiddusha Rabba – the great kiddush – for a blessing that simply consists of Borei pri ha-gafen seems a bit odd. Rashi and the Rashbam explain that it refers to the fact that Borei pri ha-gafen is a much more common blessing than kiddush, which is said only once a week, so it is said with greater frequency. According to Rabbenu Yehonatan it receives that title because of the role that this blessing plays in honoring the Shabbat. The Mekhtam suggests that it is lashon Sagi Nahor – an expression used by the Talmud to suggest the opposite of its simple meaning. Since we do not want to “belittle” this very simple blessing we switch its name to “the great kiddush.”