Observed on a weekday following the Day of Pentecost
Almighty and everliving God, who through the Book of Common Prayer restored the language of the people in the prayers of your church: Make us always thankful for this heritage; and help us so to pray in the Spirit and with understanding, that we may worthily magnify your holy Name; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.
In 1549, under the reign of Edward VI, successor to Henry VIII, the primary language of public worship in England and other areas ruled by Edward was changed from Latin to English, and the first Book of Common Prayer came into use. It was first used on Pentecost Sunday, 9 June 1549, and the occasion is now commemorated "on the first convenient day following Pentecost." The Book was the work of a commission of scholars, but primarily of Thomas Cranmer, Archbishop of Canterbury. More information on The Book of Common Prayer – 1549, including a facsimile copy in pdf can be found here - http://justus.anglican.org/resources/bcp/1549/BCP_1549.htm