His day was complete, fulfilled. He can look back on that day and say that it was a very good day. The fact that he had seen Kennedy made it a very good day for him (at the time, of course). Sometimes we say it sarcastically. If a number of bad things happen and one final disaster occurs, we might say sarcastically, "Oh, great! This just makes my day." In the example above, the meaning is not sarcastic. The agent, at the time, was quite happy with how his day was going.

thank you!

We usually call him by his full name 'Lee Harvey Oswald.' You know, he was a Marine, and I think tried to go the Soviet Union (before killing Kennedy) but they didn't want him. Also: we sometimes or often refer to certain presidents by the full initials: Kennedy is JFK, Franklin Delano Roosevelt is FDR, Teddy (Theodore) Roosevelt is sometimes TR, Lyndon Bates Johnson is LBJ. I think the Dutch pronunciation of Roosevelt is different from the way many Americans say it. We pronounce the first syllable 'roe', not 'roo'. At Vietnam war protests, people would chant, "hey hey LBJ, how many kids did you kill today?". George W. Bush is often just 'Bush', and George Herbert Walker Bush is often 'Bush senior.'. With the presidents whose initials we don't use, we often just refer to them by their last names, and we don't always use the initials when talking about them. Hope this helps, even though it's not what you asked about. Oh yeah, and I've been to the street in Dallas. Tx. where Kennedy was assassinated.