Two days after the Jews crossed the Sea of Suph, they arrived in Marah. There they received several mitzvot. One of them was to observe the Shabbat:
“There HaShem gave them statutes and laws” – including the mitzvah of Shabbat (Exodus 15:25; Talmud, Sanhedrin 56b)
In fact, the Jewish people had observed Shabbat even in Mitzrayim (Egypt). The Midrash relates:
“Moses saw that they had no rest [in Mitzrayim], so he went to Pharaoh and said: ‘If one has a slave and he does not give him rest one day in the week, the slave will die. These are your slaves—if you do not give them one day a week, they will die.’ Said Pharaoh: ‘Go and do with them as you say.’ So Moses ordained for them the Shabbat day for rest.” (Shemot Rabbah 1:32)
However, this time, in Marah the Shabbat was given to the Jewish people as a mitzvah from HaShem.
The first such official Shabbat was observed on the 24th of Nissan.
This year (5783) the upcoming Shabbat also falls on the 24th of Nissan.