The Book of Common Prayer's reading schedule is based on what's called the church calendar, or the liturgical year (Liturgy basically means, 'pattern of worship').  From early on in the history of the church, communities of followers of Jesus-particularly communities of monks and nuns-began to structure their common life of prayer, worship, and reading around the major events in the life of Jesus.  Each year is divided into alternating seasons of fasting and feasting which take us from anticipation of Jesus' birth to celebration of his resurrection and ascension.
We're currently entering Easter Season, the biggest feasting season of the church year.  Easter Season is all about celebrating Jesus' victory over sin, death, and the devil.  There are three highlights of the season:

1. Easter Sunday-the commemoration of Jesus' resurrection from the grave;
2. Ascension Day-the celebration of Jesus' ascension into heaven, to sit enthroned at his Father's right hand as the rightful ruler of heaven and earth;
3. Pentecost Sunday-the celebration of the day on which Jesus fills his followers with the Holy Spirit, empowering them to spread the news of his victory to the rest of the world.  This moment is arguably the climax of Jesus' life work; more than once, Jesus says that the reason he lived and died was to gain the authority to fill his followers with the Holy Spirit.

Easter season starts, of course, with Easter.  It lasts seven weeks, imitating the biblical story, in which the resurrected Jesus spends forty days with his followers before ascending into heaven, and then sends the Holy Spirit to fall on his followers ten days later, during the feast of Pentecost (which means 'fiftieth day').

During the first week after Easter, on Ascension Day (the 40th day after Easter), and on Pentecost Sunday, some or all of the readings depart from their normal schedule to match the day's theme.