On Sivan 3, G-d instructed Moses to "set boundaries for the people around, saying, 'Beware of ascending the mountain or touching its edge...'" (Exodus 19:10-12) in preparation for the Giving of the Torah on Mount Sinai three days later. To this day, we mark the "Three Days of Hagbalah ('Boundaries')" leading to the Giving of the Torah on Sivan 6.
In his advance towards the destruction of Jerusalem, Rome Emperor Titus Flavius Vespasianus ("Vespasian") captures Jericho and massacres all its inhabitants.
On 4 Iyar, 4925 (1165), Maimonides sets sail from Fez, Morocco, to escape Islamic persecution. The journey is fraught with danger, including a storm on 10 Iyar that threatens to capsize his ship. Finally, on 3 Sivan, Maimonides arrives safely in Acco, Israel. He establishes this date as a day of rejoicing, festivities, and gifts to the poor, to be kept by him and his descendants until the end of time (Charedim ch. 65 [5744 ed.).
Link: Rambam (Maimonides)
Today begin the three days of preparation for the festival of Shavuot known as the "Three Days of Hagbalah" (see today's "Today in Jewish History"); in the custom of certain communities, the mourning practices of the Omer period, such as not to hold weddings or get a haircut, are now suspended.
Tomorrow is the forty-eighth day of the Omer Count. Since, on the Jewish calendar, the day begins at nightfall of the previous evening, we count the omer for tomorrow's date tonight, after nightfall: "Today is forty-eight days, which are six weeks and six days, to the Omer." (If you miss the count tonight, you can count the omer all day tomorrow, but without the preceding blessing).
The 49-day "Counting of the Omer" retraces our ancestors' seven-week spiritual journey from the Exodus to Sinai. Each evening we recite a special blessing and count the days and weeks that have passed since the Omer; the 50th day is Shavuot, the festival celebrating the Giving of the Torah at Sinai.
Tonight's Sefirah: Yesod sheb'Malchut -- "Connection in Receptiveness"
The teachings of Kabbalah explain that there are seven "Divine Attributes" -- Sefirot -- that G-d assumes through which to relate to our existence: Chessed, Gevurah, Tifferet, Netzach, Hod, Yesod and Malchut ("Love", "Strength", "Beauty", "Victory", "Splendor", "Foundation" and "Sovereignty"). In the human being, created in the "image of G-d," the seven sefirot are mirrored in the seven "emotional attributes" of the human soul: Kindness, Restraint, Harmony, Ambition, Humility, Connection and Receptiveness. Each of the seven attributes contain elements of all seven--i.e., "Kindness in Kindness", "Restraint in Kindness", "Harmony in Kindness", etc.--making for a total of forty-nine traits. The 49-day Omer Count is thus a 49-step process of self-refinement, with each day devoted to the "rectification" and perfection of one the forty-nine "sefirot."
Links:
How to count the Omer
The deeper significance of the Omer Count
Tachnun (confession of sins) and similar prayers are omitted from the prayer service.