Are various times in our lives we Catholics are asked to produce a Certificate of Baptism. Many people are unaware how to go about this. I am grateaful to Father Timothy Finnegan of The Hermeneutic of Continuity Blog http://the-hermeneutic-of-continuity.blogspot.com/ for the following helpful information.
Every parish keeps a permanent register of Baptisms. This is one of the most important things that the parish possesses. If things are done properly, your confirmation should also be recorded and also your marriage (or religious profession, or ordination.)
A "Baptism Certificate" is a copy of the entry that is made in the Baptismal register. If you are getting married, you need to supply a recent baptismal certificate (within six months of the wedding) which will then also record that there are "no other entries" (i.e. no previous marriage recorded) and that you are "free to marry". It is also OK to ask for a Baptismal certificate if you just want to know when your baptism was.
If you want to get a certificate of Baptism, send a letter addressed to the parish priest of the parish where you were baptised. Give your full name (as it was at the time of your baptism if your name has subsequently changed.) Give your date of birth and roughly when the Baptism took place. If you were baptised as a baby it is sufficient to say this but if you were baptised at three or four years old, it will help the parish priest if you tell him this; and enclose a stamped addressed envelope just to make things a little easier!