We are in the midst of the 49 day "countdown" that runs from the second day of Passover until the holiday of Shavuot ("Weeks" or "Pentecost"). This seven week period is called "Counting the Omer" (סְפִירַת הָעוֹמֶר) in Jewish tradition (Lev. 23:15-16). During the Temple period, on each of these days an appointed priest would wave a sheaf (omer) of barley before the LORD as a symbolic gesture of dedicating the coming harvest to Him. This ceremony was called tenufat ha-omer ("waving of the sheaf"). On the 50th day, however, a sample of the first crop of the wheat harvest was baked into two loaves of leavened bread (called "Shtei Ha-Lechem," שְׁתֵּי הַלֶּחֶם) and waved before the altar as the climactic rite of the season (Lev. 23:15-20). Notice that this was the only time that otherwise forbidden chametz (leaven) was used by the priests for the avodah (see Lev. 2:11).
The 49 days between Passover and Shavuot is the chain that links the two festivals together, and indeed Shavuot is regarded as the culmination of the experience of redemption, sometimes called Atzaret Pesach, the "Conclusion of Passover." Just as the redemption by the blood of the lambs led to Israel's deliverance and the giving of the Torah at Sinai, so the redemption by the blood of Yeshua led to the world's deliverance and the giving of the Holy Spirit at Zion... And just as the covenant at Sinai created the nation of Israel, so the covenant at Zion created the worldwide people of God, redeemed from "every tribe and tongue" (Rev. 5:9). The waving of the "two loaves" of leavened bread therefore prophesied the creation of the "one new man," both Jew and Gentile, that would be "firstfruits" of the Kingdom of God. Ultimately there will be one flock, and one Shepherd (John 10:16).
Since the resurrection of Yeshua occurred during the first day of the omer (i.e., on Reshit Katzir), and the Holy Spirit was given to the disciples 50 days later on Shavuot (Pentecost), all of Yeshua's post-resurrection appearances occurred during the days of the Omer count.
Some of these appearances were as follows: On the first day of the Omer, Yeshua appeared to Mary Magdalene (Mark 16:9; John 20:16-18), some other women (Matt. 28:5-10), and then to Simon Peter (Luke 24:34; 1 Cor. 15:5). On the second day, He appeared to the two on the Road to Emmaus (Luke 24:13-32) and later that evening to the twelve disciples (Mark 16:14; Luke 24:33-39; John 20:19). A week later, He appeared to the Twelve again (John 20:26) and eight days later appeared to Thomas (John 20:24-29). Some time later, He appeared the third time to the disciples as they went back to their fishing jobs (John 21:1-14).
Still later Yeshua appeared to 500 (1 Cor. 15:6) and then to James, the half-brother of Yeshua (1 Cor. 15:7). On the 40th day of the Omer (Iyyar 25), Yeshua ascended into heaven from Bethany and commanded His followers not to leave Jerusalem until the promise of the Father was fulfilled during Shavuot (Luke 24:44-53; Acts 1:9-12).
In some Christian traditions, the "ascension of the Messiah" is celebrated 40 days after Yeshua's resurrection from the dead, about a week before the appointed time of Shavuot ("Weeks" or "Pentecost"), that is, on the 40th day of the Omer Count (on Iyyar 25).
The ascension of Yeshua was foreshadowed by Moses and the receiving of the Torah at Sinai: Just as Moses had waited 40 days before the Torah was given to Israel, so the disciples waited 40 days before the promise of the Holy Spirit was given (Exod. 24:18; John 16:7, Acts 2:1-4). In both cases, at the appointed time revelation was given -- first in the form of "the Voice of the Living God speaking from the midst of the fire" at Sinai (Deut. 5:26), and later in the form of "tongues of fire" given in glory and power. Yeshua's Voice is now heard from the midst of the fire given at Zion...
Recall that Yeshua told His followers that it was good that he would leave them, so that the Holy Spirit (רוּחַ הַקּדֶשׁ), the "Comforter" or "Advocate" (παράκλητος), would be given to them. "But I tell you the truth, it is for your advantage that I am going away. For if I do not go away, the Advocate (ὁ παράκλητος) will not come to you, but if I go, I will send him to you" (John 16:7). Notice that the word translated as "advantage" here is the Greek word συμφέρω (from σύν, "with" and φέρω, "to carry"), which suggests that we would be given power that "carries us" with the Lord during the trials of this life... Bo, Ruach Elohim: "Come, Holy Spirit..."
The resurrection and ascension make us "alive together" with the Messiah. Because of Yeshua, we are able to live on a different level of reality, free from the "law of sin and death" (Rom. 7:4, 8:2). We now have a new and living relationship with God - based on the freedom and intimate revelation given by the Holy Spirit - all because of our ascended Lord Yeshua, blessed be He.