September is the month where everyone collectively decides summer is over, whether the weather agrees or not. Labor Day closes the unofficial summer on the first Monday, the autumn equinox resets the light around the 22nd or 23rd, and Hispanic Heritage Month kicks off on the 15th and runs into October. It's also when the NFL season starts, which for a large fraction of the country counts as a real holiday.
Every day in September
✝️ Feast of Creation (Christianity)
🗳️ Democracy Day (Tibet)
🕊️ China's victory over Japan commemoration related observances: Armed Forces Day (Republic of China)
🎊 Immigrant's Day (Argentina)
🍂 Earliest date on which Jeûne genevois can fall, while September 11 is the latest; celebrated on Thursday after the first Sunday of September. (Canton of Geneva)
💃 The earliest date on which the Abbots Bromley Horn Dance is performed
🎊 Air Force Day (Pakistan)
♂️ Christian Feast Day: Adrian and Natalia of Nicomedia (Roman Catholic Church)
🎊 Armored Forces Day (Ukraine)
🏛️ Amerindian Heritage Day (Guyana)
🎊 Battle of Tendra Day (Russia)
🕊️ Commemoration of the mass hanging of the Saint Patrick's Battalion (Mexico)
🎊 Day of the Programmer, during a non-leap year. (Russia)
⚙️ Engineer's Day (Romania)
🎊 Battle of Britain Day (United Kingdom)
🗽 Cry of Dolores, celebrates the declaration of independence of Mexico from Spain in 1810. See Fiestas Patrias
⚓ Australian Citizenship Day
🎵 Day of National Music (Azerbaijan)
🎖️ Armed Forces Day (Chile)
📜 Constitution Day (Nepal)
🌷 Autumnal equinox observances in the Northern Hemisphere, vernal equinox observances in the Southern Hemisphere (see September 22): Spring Day (Argentina)
♀️ American Business Women's Day (United States)
🎊 Grito de Lares (Puerto Rico)
🎖️ Armed Forces Day (Peru)
✊ Armed Forces Day or Revolution Day (Mozambique)
🎊 Day of the National Flag (Ecuador)
✒️ Consumación de la Independencia (Mexico)
⭐ Czech Statehood Day (Czech Republic)
🎊 Inventors' Day (Argentina)
🏛️ Agricultural Reform (Nationalization) Day (São Tomé and Príncipe)