The late spring Feast of Pentecost shows the harvest of firstfruits, God's church. It is a continual reminder of our part in God's plan!
The apostle Paul teaches that tongues (languages) are only used to communicate intelligently, not gibberish. Tongues originally served as a sign for unbelievers.
How does one count to Pentecost when Passover is on a weekly Sabbath, making the Last Day of Unleavened Bread the only other available Sabbath to begin the count?
Jesus Christ's and Paul's example in Sabbath observance (including the annual Sabbaths) provide a model as to how we keep the Sabbath and the holy days.