According to Moses, Pentecost was observed by the bringing of new offerings, on the fiftieth (pentecoste) day after the day after the Sabbath following the Passover. So Jesus (our Passover) instituted the Holy Sacrament at twilight before His crucifixion, rested in the tomb on the Sabbath (Saturday), and rose from the grave on the day after. Fifty days later comes Pentecost on which a new offering is given, that is, a new age of the Spirit inaugurated. Today Christ's promise to His disciples is fulfilled, as the Holy Ghost, the Comforter, is sent to them, and they begin to declare His peace to the nations assembled there, on the fiftieth day from Easter. Pentecost, for seven is creation=s number, and forty-nine is creation squared, creation fulfilled; fifty then begins the age of the new creation. Sunday, because this is the third and final great Sunday of history, Creation Sunday being the first and Easter Sunday the second. So on Pentecost Sunday all is fulfilled, when the Spirit begins to give life to the Church through apostolic preaching.
--Burnell F. Eckardt Jr., Every Day Will I Bless Thee (Sussex, Wis.: Concordia Catechetical Academy, 1998), s.v. The Feast of Pentecost. To Purchase this book for $21.00, go pages 2-3 of the online catalog for the Concordia Catechetical Academy for ordering information, then click here for their printable order form to mail in your order (their online bookstore is currently under construction).