The 155th mitzvah is that we are commanded to make a verbal declaration when Shabbos enters and when it leaves. We must mention the greatness and exalted character of this day, and how it is distinct from the other days of the week which precede it and follow it.
The source of this commandment is G‑d's statement1 (exalted be He), "Remember the Shabbos day to sanctify it." This means that one should commemorate it through mentioning its holiness and greatness. This is the mitzvah of Kiddush.
The Mechilta says the following: " 'Remember the Shabbos day to sanctify it,' — this means one should sanctify it through reciting a blessing." The Sages said explicitly,2 "Remember [Kiddush] over wine."
The Sages also said,3 "Sanctify it when it enters and when it leaves," [the phrase, "when it leaves"] referring to Havdalah, which is also part of our commandment to remember Shabbos.
The details of this commandment are explained in the end of tractate Pesachim,4 and in many places in Berachos5 and Shabbos.6
