Laudator Temporis Acti

Online Etymology Dictionary, s.v. Pentecost:

O.E. Pentecosten "Christian festival on seventh Sunday after Easter," from L.L. pentecoste, from Gk. pentekoste hemera "fiftieth day," fem. of pentekostos, from pentekonta "fifty," from pente "five." The Hellenic name for the O.T. Feast of Weeks, a Jewish harvest festival observed on 50th day of the Omer (see Lev. xxiii:16).

In a very interesting post, Balashon - Hebrew Language Detective discusses the word hamsin and states:

The weather in Israel at this time is often characterized by hot winds .... The word hamsin in Arabic actually means fifty. This derives from an Arabic tradition that there are 50 days of these hot winds.

It is probably just a coincidence that the arrival of the Holy Ghost on Pentecost was accompanied by wind and fire (Acts 2.1-4):

And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.