The Chronological Order of Ponies · 6:01pm Jul 7th, 2015
This has been on my mind for awhile now so I'm just going to put it out here. I've heard many that state the show confirmed that the four seasons happen all in a year. So like any logical person I looked at the sources and read the transcript to see where it was spoken. What I found were pretty vague quotes.
Princess Celestia: I must admit that it is wonderful to actually be looking forward to the Summer Sun Celebration. [...] For my subjects, it has always been a celebration of my defeat of Nightmare Moon. But for me... [sighs] It was just a terrible reminder that I'd had to banish my own sister. [...] But now it has become a wonderful reminder of her transformation back into Princess Luna and our happy reunion.
Princess Celestia: Citizens of Equestria, it is no longer with a heavy heart but with great joy that I raise the summer sun. For this celebration now represents not the defeat of Nightmare Moon, but the return of my sister, Princess Luna.
For one speeches are generally prescript, so it is very likely that she said the exact speech before. Much like how High School Grduations have the same speech every year. Another point is that there is never a instance that she stated that this is the first year. For it could just be a general statement and not necessarily mean it happened recently. For example if my brother joined the army, and then after several years came home, I could say "It's great to have my brother around again" whenever I wanted. It wouldn't mean he'd only been home for a week or so, I could say it years later. If this was real It's would mean that Fall and Winter happens twice in one year.
So that fact that people are latching onto that vague and pointless line of dialogue as flawless evidence is a bit puzzling. Granted the chronological order in any fandom is built on assumptions and guessing so I get it is very hard to make a solid timeline. So I'm not going to make a big deal out of it, after all we all have our own theories.
I'd normally agree with you, and for a time I did. But look at the Season 5 episode Tanks for the Memories. Rainbow Dash goes through the Five Stages of Grief because she won't be able to share winter with Tank (a bit of an overreaction if you don't realize it was actually a nice allegory for death). Twilight is clearly already an alicorn in that episode (so it's post-Season 3), and Rainbow took him into her home in Season 2, so if each season (or even each pair of seasons) was its own self-contained (albeit anachronistic) year, this should have come up much sooner.
So May The Best Pet Win had to have happened sometime in the spring right before Magical Mystery Cure. That's pretty late in the timeline for an early Season 2 episode if we were to split Seasons 1-3 into 2-3 years, if you ask me. Plus, this confirms that Season 4 took place in the span of a few short months (I think we saw a shot of the Friendship Castle in Tanks for the Memories), so while there is some deniability, it's not too big a stretch to reason that Seasons 1-3 are all crammed into a single year.