along with what puzzleannie said (especially noting the liturgical calendar as sanctifying each day to God and being a sign of unity in the universal Church) there are a few other things to note that are instructive to people who are new to the Church.
the liturgical calendar has its origins in the Jewish roots of Christianity. the Jews observed a detailed calendar much the same way we do. this is illustrated throughout Scripture, including the Gospels. there was yearly observance of Passover, the Day of Atonement, Feast of Booths, etc.
early in Christian history, significant celebrations were added into that calendar by the first generations of Christians, sundays, Easter, etc. as the Christians separated themselves from Judaism, the feasts and solemnities also became decidedly Christian. only those that signified Christ and the New Covenant remain. many of our solemnities are the Christian version of days significant to the Jews (Holy Thursday, Pentacost, et al.)
over time, the calendar developed. special days were added in order to deepen the understanding of the Christian faithful. whatever proved to be a benefit to faith eventually became ratified for the universal Church by councils and the popes. many of these came out of the pious practices of local Churches. some were just introduced as a new practice. as far as i know, the seasons of the calendar are so ancient that they are from the undocumented development of the Church in the era of the apostles and the church fathers.