Weather data overlays are a really useful way to see how your buildings' energy consumption fluctuates with weather patterns such as temperature, wind speed, and rainfall.
If the Analytics page is not allowing you to overlay weather data, you may be trying to apply the overlay to multiple meters in different locations. You can only overlay weather data for meters that are in the exact same geographical location.
It's also important to know that if you have the data granularity set to automatic, you can only overlay weather for periods of less than 31 days. If you have the granularity set to hourly or half-hourly, you can overlay weather for any period of time.