Every fan of any sports team has a special moment, an amazing game, or a whole magical season that's indelibly etched in their memories. In my lifetime alone, as far as whole seasons go, there's been the 1969 Mets (just barely in my lifetime, but still), the 1972 Dolphins, the 1983-88 Edmonton Oilers, the Showtime-era Lakers, the 1991-92 Chicago Bulls, the 2001 Mariners, and there are probably more from other sports or teams that I've left out.
For Arsenal, though, that magical season was 2003-04. As even the most casual fan of Arsenal or the Premier League probably knows, Arsenal's season went like this: P38 W26 D12 L0. It was and still is the only unbeaten campaign in modern English soccer history, and it came with a title that Arsenal clinched at the best possible venue to do such a thing (coincidentally, the first Premier League championship ever to be won at a condemned building site) with four games to spare.
Draws against Birmingham City and Portsmouth sent Arsenal into the final day of the season needing a result to maintain their unbeaten record. And on May 15, 2004, Arsenal got that result, beating Leicester City 2-1 at Highbury to emphatically close out the best season in modern English soccer history.
It's not exactly a slow news week for Arsenal this week, but there are certain things that need to be commemorated when they come back around, and the anniversary of becoming Invincible (or as Wenger put it, "immortal") is certainly one of them. As with any season like this, you don't really realize how fully special it is until it's almost over - the week-to-week stress of watching your team try to win kinda takes over, and it's not until about the six week to go stage that you think "hey, something amazing's happening here".
Once Arsenal clinched the title that year, they had nothing left to play for, but I am very happy that they decided to keep playing for wins anyway - seasons like that don't happen very often, and when you're given a shot at history, you gotta take it. They did, with style, and that Invincible Arsenal team gave us all a pretty rare treat on May 15, 2004.