For those who are sincerely wrestling with the timing of the 50 days leading up to Pentecost, here is the Ferrar Fenton Bible's translation of Leviticus 23:15-17:

"You shall also count for yourselves from the day after the Sabbath that you bring the Wave sheaf, seven Sabbaths. They must be complete.  Then after the seventh Sabbath, you shall count fifty days, when you shall bring a new offering to the Ever Living.  You shall bring from your dwellings two wave cakes of two tenths of fine flour.  They shall be fermented - baked in an oven for the Ever Living."

Even though you start day one, of the 50 day count, right after the complete seven Sabbath weeks, that first day of the 50 day count is not one of the Moed's in Leviticus 23!  Even though, traditionally, it became the day to offer the first fruits of the "planted in the prior late fall" wheat, it was not to be the timing for the wave offering involving the two loaves.

The wave offering of the two loaves were a first fruits offering of the wheat that had just been planted earlier, that spring.  It takes approximately four months for the spring planted wheat to be ready of harvest, hence the offering of the wave loaves made from the that very first fruits grain, of that "second" wheat harvest, during the first days of the 5th month, Av.  It was also the timing of the first ripe grapes, in time to make new wine for Pentecost as well.

Interestingly, this placement of Pentecost "fully coming" during the first part of the 5th month places it at a midpoint between the second Passover and Tabernacles.  Think here of the seven branch Menorah.  When Jesus is said to be standing in the midst of the lampstands when He revealed Himself to John in the first chapters of Revelation, I believe it is a hint for Pentecost.  And, as Jesus fulfills the Moeds of God in our near future, it is also a hint that the second Passover also will be very signficant.

When Moses gave a cliff-notes version of the Feasts in Deuteronomy 16, he did not give the detail about the necessary 50 days unit of time.  That is correct.  Do we then presume that such is not to be included in determining the correct timing of Pentecost?  Well, by that logic, what does one do with the Feast of Trumpets and also Yom Kippur, the Day of Atonement, when such were also not included in Moses' summary?

Was Israel wrong to bring a first fruits of the "planted in the late fall/winter" wheat on the day one after the seven complete Sabbath weeks were accomplished, and also considering that event as one of the three annual times the males were required to gather and appear before God at His tabernacle/temple?  I do not believe so.  However, what it does is it leaves the true Pentecost timing as a bit of a mystery, which I think God ultimately intended.  Pentecost ends up being a Feast, a Moed, that is actually to be in the forefront thinking of the Body of Christ.  What is ironic to me is that, generally speaking, the count of the true Pentecost has ended up being also a mystery to the Body of Christ today.

For what it is worth.