Most of the special events are actually based on real life holidays or events though, both Western/US festivals like Starlight Celebration (Christmas), Valentione's Day (Valentine's Day), Egg Hunt (Easter) and All Saints' Wake (Halloween), and Japanese festivals like Heavensturn (New Year), Little Ladies' Day (Doll Festival) and Moonfire Faire (Tanabata/summer) - the only seasonal event that isn't is the Rising that celebrates the game's real-life rebirth from the 'ashes of 1.0' (and in story, the restoration of the world following the Calamity). All these events are run at the times corresponding to the real world and have no actual in-game bearing on the Eorzean calendar.

This is because story-wise Hydaelyn exists in a kind of 'time bubble' where the passage of time is only noted to occur if necessary for the plot. Otherwise, despite regular seasonal events happening as the real world calendar proceeds, there is no actual meta connection to the time and date they occur in real life and in Eorzea.

Technically, Hydaelyn is still in the year 1 7th Astral Era and it is still five years since the Calamity occurred, despite the constant progression of seasonal events from year to year. A little suspension of disbelief is needed here.