In 3333/428 B.C.E., Yechezkel Hanavi received a nevuah at the river of Chebar about Maaseh Merkavah. That perek is read as the haftarah on Shavuos.Let me unpack that a bit. Tammuz is the fourth month of the Jewish calendar and the fifth day of that month varies from year to year in the secular Gregorian/civil calendar, but this year it happens to be today. On that date Ezekiel the prophet received a prophecy at the river of Chebar about "the working of the Chariot." That section is read as the synagogue reading from the prophets on the Sabbath of the relevant week — i.e., this week.
The passage in question is Ezekiel chapter one.
The phrase Ma'aseh Merkavah means "the working of the Chariot" or "the matter of the Chariot," or the like. The "Chariot" is God's throne, which also has wheels according to Daniel 7:9 and related Second Temple Jewish visionary literature, including 1 Enoch (the Book of the Watchers) 14:18 and the Book of Giants (see A.12). These texts are inspired by Ezekiel's Merkavah vision, but the placement of the wheels in it is ambiguous. Ezekiel's vision, at least in the Hebrew text, does not use the term "merkavah"/"chariot."
The dating of the event in the quote above follows the traditional Rabbinic chronology, which at that point is off by about a century and a half from the modern scholarly reconstructed chronology (which is based on excavated cuneiform material and is highly reliable). Ezekiel's vision actually took place in 593 BCE on the fifth of Tammuz.
Ezekiel's Merkavah visions in the Book of Ezekiel chapters one and ten were fundamental inspirations behind much of Western mystical literature, the best-know example of which is perhaps Revelation 4 in the New Testament. But they also heavily influenced the mystical work of Pseudo-Dionysius the Areopagite, which was a major wellspring of subsequent Christian mysticism. Ezekiel's visions are also the fundamental inspiration of Jewish Merkavah mysticism as found in the Hekhalot literature, on which much more here, here, here, here, here, and links. Merkavah mysticism was in turn a major influence on the medieval Jewish mysticism of the Kabbalah.