Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 5. Christian Worship” (Includes an introduction and sections 59-74).
The next festival after Passover and hence, after Easter, was Pentecost. Schaff notes this as originally a feast of harvest. “It was universally observed, as early as the second century, in commemoration of the appearance and heavenly exaltation of the risen Lord, and had throughout a joyous character. It lasted through fifty days - Quinquagesima - which were celebrated as a continuous Sunday, by daily communion, the standing posture in prayer, and the absence of all fasting” (Schaff 2014, Loc. 15059). Readings focused on the Book of Acts. Schaff notes that later the Ascension, on the fortieth day, then Pentecost day proper, also called Whitsunday became the main features. Pentecost became the most common day for a baptism. Spotts notes that in more recent times, Easter has expanded to cover the time to Pentecost Sunday, with observances of Pentecost beginning on the fiftieth day.