The happiest day in Remus's life was when he received his Hogwarts letter.

He knew the odds of being a werewolf and he was fully aware that no one would accept him, a threat to student's safety, in Hogwarts but Dumbledore arranged it all for him to come and study.

He was enthralled when he got the letter and was extremely grateful to Dumbledore as well. 

Later on, this was the memory he had used to create his first Patronus.