In Leviticus 23:15-16, the LORD commanded the Israelites to count fifty days from Pesach (Passover) to reach the festival of Shavu’ot (“Weeks”).
That’s seven full weeks—forty-nine days—between the two festivals, with the fiftieth day being Shavu’ot. The Greek word for “fifty” is pentekonta, so the festival also became known as Pentecost.  In Exodus 23:16, Shavu’ot is referred to as the Feast of the Harvest, indicating the time of the late spring grain harvest—the last of the spring festivals.
In the days of the Tabernacle or (at other times) the Temple, on the first day of the Omer the Israelite priests would wave a sheaf (omer)—a tied bundle of harvested cereal including both stalks and heads of grain—before the LORD in the Holy Place. This is called s’firat ha-omer (counting the sheaves). Instead of referring to the practice as “counting the days,” we talk about “counting the Omer.”
Since the Second Temple in Jerusalem was destroyed, Jewish tradition developed so that the days are counted each evening in a particular way. Believers from the nations (non-Jews like myself) may join with all Israel in the ritual, though with a little modification. (After all, in joining ourselves with the Messiah of Israel, we have become members of the greater commonwealth of Israel.) Here are the steps for counting the Omer:
- Jews proclaim the blessing: “Blessed are you, LORD our God, King of the universe, who has sanctified us by his commandments and commanded us concerning the Counting of the Omer.” (Gentiles should not recite this. )
- Declare the count: “Today is forty-one days, which is five weeks and one day, of the Omer.”
- Pray for the Temple, the “Holy House,” to be rebuilt: “May the Merciful One restore unto us the service of the Beit HaMikdash to its place, speedily in our days. Return your Dwelling Presence to Zion, your city, and send us Yeshua our Messiah a second time. Let him reign upon the throne of David in Jerusalem, your holy city. Amen, Selah.” 
- Read Psalm 67.
- Pray for purification and sanctification in preparation for Shavu’ot.
Why didn’t the LORD simply specify the date on the Hebrew calendar, 6 Sivan, on which to celebrate Shavu’ot? Counting the Omer links Shavu’ot explicitly to Pesach. Counting each day is like adding a link to a chain, so if Israel skips a day, the chain is broken. In a sense, therefore, Shavu’ot is the ultimate fulfillment of Pesach. These festivals originally took place in the same year, when the Passover Exodus brought the Israelites out of Egypt to the foot of Mount Sinai where they were given the Torah fifty days later. Similarly, we also see a fulfillment of both festivals take place in the same year that Yeshua died: Yeshua became the Passover Lamb fifty days before the Spirit was given (putting the Torah on the hearts of the house of Israel). 
The explicit connection between the spring festivals in the same year gives reason to suspect that the fall/autumn festivals will also find fulfillment in the same year, starting with Yeshua’s return at the sound of the great trumpet on Rosh HaShanah (the Festival of Trumpets) followed shortly after by Yom Kippur (the Day of Atonement), and culminating in Sukkot (the Festival of Booths). With this in mind, the reading of Psalm 67 every day whilst counting the Omer seems to have a special significance since it speaks of the LORD’s salvation reaching every nation. In fact, it uses three different words for “people” or “nation”: l'om, am, and goy. It is not sufficient for the LORD to be the God of Israel alone; he is indeed the God of every nation!
Tips for beginners
- Do what you can manage, and don’t worry about what you can’t. Gentiles are not commanded to count the Omer; we do it to identify with Israel and to build anticipation for Shavu’ot.
- On difficult days, doing at least step 2 will suffice to keep the chain unbroken. It only takes a few seconds.
- Set a reminder on your phone to count the Omer each evening, starting after Pesach. In 2017 you should start counting after sundown on the evening of April 11, with the last count (day 49, on the eve of Shavu’ot) on May 30.
- Acts 2:1, 20:16; 1 Corinthians 16:8
- The commandment was given to Israel, not to the nations. However, Gentiles may take a moment to reflect on the blessing, sanctification, the commandment, its faithful fulfillment by the Jewish people, and our unity with Israel in Messiah. Gentile believers may even wish to recite a modified form of the blessing, as follows: “Blessed are you, LORD our God, King of the universe, who has sanctified Israel by his commandments and commanded Israel concerning the counting of the .”
- The second part of this prayer (“Return… holy city.”) is not the traditional Jewish prayer. It is a petition written by Messianic luminary, Yechiel Lichtenstein.
- Jeremiah 31:33; Hebrews 10:16