The whole country and Moscow live in a certain seasonal rhythm or cycle. The cycle is signified by repetitive events, weather conditions, people activities, things that happen each year over and over again.
September means vacations are over and people get back to Moscow. Many return from their dachas if they spent whole summer there. In practical terms this means more car traffic and more people on streets and in subway. More rush. People go to supermarkets to get stock of groceries for their first weeks after vacation. Lines in grocery stores become notably longer. Car traffic gets worse.
September 1st is a holiday (not a bank holiday) - “The day of knowledge”. School year starts September 1st and most colleges and universities start first semester after summer break beginning of September. Parents prepare their kids for upcoming school year. The preparation means buying school necessities such as notebooks, school appliances, outfits, whatever else kids might need for school. In this period stores are packed with parents and their kids. All shopping malls are crowded, no parking places, long lines for fast food counters.
Locally grown seasonal fruits and vegetables start to appear: apples, pears, watermelons, plums, corncobs, pumpkins. Fruit seller says they will persimmons very soon. Days are getting shorter. Outside temperatures are slowly going down and soon we will be missing warm days. It rains very often. Only in September it becomes apparent, how short Moscow summer really is.