What does Whitsun Day mean in the Bible?

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The American Church Dictionary and Cycopedia - Whitsun Day
A high Festival observed in the Church on the fiftiethday after Easter, in commemoration of the outpouring of the HolySpirit upon the Apostles on the day of Pentecost as "they were allwith one accord in one place" in Jerusalem. Whitsun Day isthe Birthday of the Christian Church, and as such it has beencommemorated for nearly two thousand years by Christian people andobserved by them with holy joy and deep thanksgiving for thefulfilment of our Lord's promise to send the Comforter to Hiscomfortless people.
By the devotions of Whitsun Day we have brought to our remembrance,in the most beautiful and striking manner, the operations of God bythe Spirit's power. By Proper Psalms, Proper Lessons and EucharisticScriptures, and by Proper Preface in the Communion Service, we learnhow that in the Holy Ghost and His Presence in the Church we havethe great power and renewing grace of God made availing to us. Theecclesiastical color is red as symbolical of the "cloven tongueslike as of fire," in which form the Holy Ghost lighted on the headof each of the Apostles. (See HOLY GHOST.)
As to the derivation of the word "Whitsun" there seems to be greatuncertainty and difference of opinion. Some derive it from theword white, shortened to "whit," in reference to the diffusionsof light and knowledge which on this day were shed upon the Apostles,in order to the enlightening of the world; also in reference tothis being the time of Baptism in the ancient Church, each candidatebeing clothed with white garments. Others derive it from the oldSaxon word wit, meaning wisdom which is the special gift of theHoly Ghost. Again others derive it from the word Pentecost, theoriginal name of the Festival, through the German Pfingsten, hencePingsten, changed in the Saxon to Wingsten, and this being corruptedinto Whitsun, meaning, therefore the same as Pentecost, that is,the fiftieth day. (This last seems to be the most probable derivationas is seen in the use of the terms Whitsun Monday, WhitsunTide, etc.)
This Festival is of especial interest to Churchmen as it wason Whitsun Day, June 9th, 1549, that the Book of Common Prayer, inEnglish, was first used. "That day was doubtless chosen," says abeautiful writer, "as a devout acknowledgment that the Holy Ghostwas with the Church of England in the important work then taken.May He ever preserve these devotional offices from the attacks ofenmity or unwisdom, and continue them in that line of Catholicunity wherein He has guided the Church hitherto to keep them."

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Octave - The only Feasts mentioned in the Prayer Book,having an Octave as of obligation are Christmas, Easter, Ascensionand Whitsun Day, each being honored with a Proper Preface in theCommunion Office which is to be used each day during the week. Trinity Sunday was formerly the Octave of Whitsun Day, and probablyfor this reason its Proper Preface is not repeated during the week
Whitsun Tide - The week beginning with Whitsun Day is so called
Proper Preface - The Preface is that portion of the CommunionOffice, beginning with the words "Lift up your hearts," immediatelypreceding the TER SANCTUS (which see), and the Proper Prefacecontains the additional words set forth to emphasize the greatTruths commemorated on certain High Festivals, namely, ChristmasDay, Easter Day, Ascension Day, Whitsun Day and Trinity Sunday
Pentecost - Whitsun Day is so called, being observedfifty days after Easter, the Christian Passover, and because itwas on the Day of Pentecost that the Holy Ghost was given. (SeeWHITSUN DAY
Immovable Feasts - As some of the Feasts, such as Ascension Day, Whitsun Day,etc
Days of Obligation -   Whitsun Day Movable
Easter Even - " Baptism iswont to be administered on Easter Even, because this was one ofthe two great times for baptizing converts in the Primitive Church,the other being Pentecost or Whitsun Day
Whitsun Day - Whitsun Day isthe Birthday of the Christian Church, and as such it has beencommemorated for nearly two thousand years by Christian people andobserved by them with holy joy and deep thanksgiving for thefulfilment of our Lord's promise to send the Comforter to Hiscomfortless people. ...
By the devotions of Whitsun Day we have brought to our remembrance,in the most beautiful and striking manner, the operations of God bythe Spirit's power. )...
This Festival is of especial interest to Churchmen as it wason Whitsun Day, June 9th, 1549, that the Book of Common Prayer, inEnglish, was first used
Ember Days - The Ember Days are the Wednesday, Friday and Saturdayafter the First Sunday in Lent; Whitsun Day; the 14th of Septemberand the 13th day of December, and are regarded as the Fasts ofthe four seasons
Movable Feasts And Fasts - 30) whether before or after; the three remainingSundays in Advent; Septuagesima, Sexagesima and QuinquagesimaSundays; the Six Sundays in Lent; Rogation Sunday; Ascension Day,Whitsun Day and Trinity Sunday; Monday and Tuesday in Easter Week;Monday and Tuesday in Whitsun Week; also the number of Sundaysduring the Epiphany and Trinity Seasons is variable, these Seasonsbeing longer or shorter according to the time Easter is kept
Trinity Sunday - Trinity Sunday is a Festival of late institution,as the day on which it is observed was originally kept as the Octaveof Whitsun Day
Ascension Day - Proper Psalms, Proper Lessons and Proper Preface in theCommunion service place it on the same footing as Christmas Day,Easter and Whitsun Day
Prayer Book, the - The first practical resultof this movement is seen in the First Prayer-book of Edward VI setforth in English, and which was publicly used on Whitsun Day, June9th, 1549
Liturgy - At the time of theReformation the Liturgy after many revisions was first setforth in the English language on Whitsun Day, 1549