In the tradition of Timehop, Cakeday is a clever little application of the Internet Archive's records. Enter a Reddit username, and it pulls the account creation date, then matches it to the nearest Wayback Machine snapshot for Reddit's front page. It's effectively a record of what was going on when you joined the site, or at least what was going on for a very specific period of time within about a day of it.
My own front page evoked a little bittersweet nostalgia. People still offhandedly referenced Left 4 Dead, one of my favorite series at the time. I apparently joined around the same time as Jon Stewart and Stephen Colbert's Rally to Restore Sanity and/or Fear, which I actually do remember attending — although I can't recall whether I was in favor of sanity or fear. The rest of it could be almost any year. It's a mix of general questions ("Why do you smoke cigarettes?"), cute Halloween costumes, and hot-button political tidbits like a story about TSA scanners. Actually, despite all the recent Reddit controversy, it mostly makes me realize how much more substantial today's front page looks.
It's actually more interesting to look up other people's inaugural front pages. Here are co-founders Steve Huffman and Alexis Ohanian, way back at the site's launch in 2005. Or ex-CEO Ellen Pao's 2013 "cakeday," when people were excited about FTL and Far Cry 3. (Video games are the carbon-14 of Reddit.) Or President Barack Obama, who caught the site on a bit of a reptile binge. Or Bill Nye, who joined in the heat of the 2008 election. Because seriously, what else is a comprehensive recording of the internet for?