The Catechism (1452) says that perfect contrition needs to include the intent to go to confession “as soon as possible" for the contrition to obtain forgiveness of mortal sins. Does this mean that after such a sin, I ought to call and make an appointment straightaway, or grab the priest after Mass, or is it enough to go to the next regularly scheduled confession time?
The tricky thing about making a perfect act of contrition is that one never knows if it is perfect. So, “as soon as possible" means as soon as possible: making an appointment right away or asking the priest when you see him to hear your confession. I wouldn’t receive Holy Communion until I knew that I was in the state of grace. Otherwise, I could be offending our Lord with the mortal sin. Nothing is worth such an offense against him who loves us so much.