To give an idea about how well the calendar was 'nailed down' even quite early, here is the list of dates (and events) from an English liturgical calendar from the 700s (around AD 702). These dates would all have been announced to the townsfolk.
JANUARY 1 Circumcision 3 St. Genevieve of Paris 6 Epiphany 13 St. Hilary 14 St. Felix of Nola 17 St. Anthony, Hermit 18 St. Peter's Chair at Rome and the Assumption of Holy Mary 20 St. Sebastian 21 St. Agnes (Virgin) 24 St. Babilas, Bishop and Martyr 25 Conversion of St. Paul at Damascus 29 St. Valerius, Bishop, and St. Lucy (Virgin) at Treves FEBRUARY 1 St. Denis, St. Polycarp and St. Brigid (Virgin) 2 St. Symeon, Patriarch 5 St. Agatha 6 St. Amandus 16 St. Juliana 22 The Chair of Peter at Antioch MARCH 1 Donatus 7 Perpetua and Felicitas 12 St. Gregory at Rome 17 St. Patrick, Bishop in Ireland 20 St. Cuthbert, Bishop 21 St. Benedict, Abbot 25 The Lord was crucified and St. James the brother of Our Lord 27 The Resurrection of Our Lord APRIL 4 St. Ambrose 22 Philip, Apostle MAY 1 St. Philip, Apostle 5 The Ascension of the Lord 7 The Invention of the Holy Cross 11 Pancratius, Martyr 14 Earliest date for Pentecost 31 St. Maximinius at Treves JUNE 2 Erasmus, Martyr 8 Barnabas, Apostle 9 St. Columkill 22 James the son of Alpheus 24 Nativity of John the Baptist 29 Sts. Peter and Paul at Rome JULY 15 St. James of Nisibis 26 St. James, Apostle, Brother of John 26 St. Symeon, Monk in Syria 29 St. Lupus AUGUST 1 The Machabees, seven brothers with their mother 5 St. Oswald, King 6 St. Syxtus, Bishop 10 St. Laurence, Deacon 13 Hippolitus, Martyr 16 (Sic) [erasure] St. Mary 25 St. Bartholomew, Apostle 28 Augustine and Faustinus, Bishops 29 Martyrdom of St. John the Baptist 31 St. Paulinus, Bishop at Trier SEPTEMBER 7 Sergius, Pope at Rome 9 (Sic) Nativity of St. Mary at Jerusalem 13 Cornelius and Cyprian 15 St. Euphemia, Martyr 19 Januarius. Martyr 21 Matthew, Apostle 22 Passion of St. Maurice 24 Conception of St. John the Baptist 27 Cosmas and Damian at Jerusalem 29 St. Michael, Archangel OCTOBER 1 Remedius and Germanus 4 Sts. Heuwald and Hewald, Martyrs 14 Paulinus, Bishop in Canterbury 18 Luke, Evangelist 28 Simon and Jude, Apostles 31 St. Quintinus, Martyr NOVEMBER 10 St. Leo, Pope 11 St. Martin, Bishop at Tours 22 St. Cecilia 23 Clement at Rome 24 Crisogonus 30 St. Andrew, Apostle DECEMBER 10 St. Eulalia and seventy-five others 20 St. Ignatius, Bishop and Martyr 21 St. Thomas, Apostle in India 25 Nativity of Our Lord Jesus Christ 26 St. Stephen, Martyr 27 John, Apostle, and James, his brother 28 The Innocents 31 St. Silvester, Bishop