If you go to the end of the Baptism service and look at the Additional Directions in The Book of Common Prayer (BCP pg. 312), you'll see the days where it is especially appropriate to have a baptism.
- The Easter Vigil because Jesus' death and resurrection are what we are baptized into.
- The Day of Pentecost, the day the Holy Spirit came down on the Apostles, which we pray for in Baptism.
- All Saints' Day (or the Sunday following) because we are baptized into the Body of Christ which is the great communion of saints throughout all time and space.
- The Baptism of our Lord because we remember Jesus' baptism on that day.
- Whenever the Bishop is present because bishops follow in the line of the apostles and "guard the faith, unity, and discipline of the Church." (BCP pg. 517)
On days when we don't have a baptism, the Additional Directions (BCP pg. 312) tell us we can replace the Nicene Creed with The Renewal of Baptism Vows (BCP pg. 292). While our baptism is once and for all, there are times we need to be reminded and renewed in our call to be part of the body of Christ Jesus. The Eucharist is always a reminder and renewal of our baptism, but on these particular days listed above, it is helpful for us to be extra mindful of what our Baptism really means.