Tradition teaches us that on the Shabbat prior to the festival of Shavuot, we read the first parashah from the fourth book of the Pentateuch, Bemidbar, or the Book of Numbers (Shulchan Arukh, Orach Chaim 428: 4). While there are exceptional years in which the next parashah -Naso- is read, it is usually Bemidbar that precedes Shavuot (Beur Halakhah ad locum)
An earlier tradition mentioned in the Talmud (Megillah 31, b) was to read Bechukotai, the last parashah of Leviticus on that Shabbat before Shavuot because it mentions the curses and punishments that the people will suffer for not observing the laws about letting the land lie fallow every seventh year (Shemitah) and in fiftieth or the Jubilee Year, which is the year that follows the seventh Shemitah, which culminates the agriculture circle.
In the Middle Ages it was suggested (Hagahot Maimoniot on Hilkhot Tefilah13: 2) that the change was due to the fact that before Shavuot, the feast of the harvest and the first-fruits, when God judges and determines the goodness and abundance of the fruits that the trees provide , words of curse should not be mentioned.
Another possibility explanation of the change is that after the destruction of the Temple of Jerusalem, the ritual festivities of bringing the first fruits and celebrating the beginning of the harvest of the wheat at the Temple, were necessarily reduced. Shavuot became celebrated primarily as a reminder of the giving of the Torah on Mount Sinai instead.
The Torah was given to the people of Israel in the desert of Sinai, and this parashah tells the organizational details of how they camped and how they traveled in that inhospitable land.
The desert, a barren and uninhabitable land, is the place where Moses saw the burning bush. It is the site away from the cacophony of human vanities that often stun sensitivity to God. It is the place where the prophet Elijah listened to the voice of God as a faint whisper that rips the silence (1 Kings 19:12)