I’m puzzled by this. Looking in the CCC, I don’t see a reference to only grave sins:
Secundum praeceptum (“Peccata saltem semel in anno confiteri”) ad Eucharistiam praestat praeparationem per receptionem sacramenti Reconciliationis, quod opus conversionis et veniae prosequitur Baptismi.
Here you go, easily found on the internet, and using the Vatican site.
Canon Law 988. a member of the Christian faithful is obliged to confess in kind and number all grave sins committed after baptism and not yet remitted…
It is recommended to the Christian faithful that they also confess venial sins.
Canon Law 989. After having reached the age of discretion, each member of the faithful is obliged to confess faithfully his or her grave sins at least once a year.
You will notice that confession of venial sins is only recommended, not an obligation.