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Smart thermostats, the devices that connect to our WiFi networks, learn our usage patterns, and allow us to control the living room temperature from the coziness of the couch, are more popular than ever. Devices from Google's ($GOOG) Nest, Honeywell ($HON), and Ecobee ($PRIVATE:ECOBEE) are the most popular by far. Specifically, the Nest thermostat is often a top-200 seller at Best Buy among all of its products.

But when do people buy smart thermostats the most? It turns out they do so when they're thinking about the warmth — or chilliness — of their homes. That's right: sales for smart thermostats spike in the months of June and December, when seasonal change reminds homeowners that it's time to switch from AC to heat — or vice versa.

By tracking sales rank at Best Buy over time for the Nest Thermostat (the most-popular of the category), it's clear that sales spike in June and December. This trend is consistent going back to 2015.

Honeywell's Smart Color Thermostat, a $199 device that hasn't sold nearly as well as the Nest, has shown gradual pick-up in sales rank over time. Interestingly, and relevant to this report, is that its sales spikes occur during the coldest and hottest months of the year.

Ecobee, a small-but-might player in the smart-thermostat space, released its ecobee4 Thermostat in the summer of 2017. Its best-selling seasons so far have been — you guessed it — the summer and winter of 2018.

Smart thermostats are rising in popularity as consumers seek to connect their homes to the internet and to their digital assistants like Amazon Alexa and Apple's Siri. Bloomberg New Energy Finance forecasts that the smart-thermostat market will grow rapidly in the next five years, reaching 14-million annual sales by 2021.