September in Cuautitlán Izcalli, Mexico, brings average daily highs of 23°C and overnight lows of 13°C in the 2016–2025 historical record. Pack clothes you can layer as conditions change. The average leaves room for both warmer and cooler days, so a flexible walking plan is more useful than deciding every outdoor stop before you arrive.
In the record, at least 1 mm of rain or snow falls on 18 of 30 days on average, a daily chance of 59%. Make flexibility part of the trip from the outset. Keep time outside easy to rearrange and choose plans that still work if it rains. The monthly figures do not establish whether rain is brief, prolonged or concentrated at a particular time.
The calculated day length, sunrise to sunset, averages about 12 hours. Sunshine averages 9 hours a day. Use the daylight window to plan the length of excursions, and check local sunrise and sunset times for the dates of your stay. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
For time in town, cluster a few stops together and leave the rest of the day open. Check transport and opening hours for any plans that run late.
The following month, October, has a similar average daily high of 23°C. Its historical rain-day chance is 39%, compared with 59% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.