I am building a garden calendar app, that uses the average temperature for each month to create a calendar. Are there websites that allow me to easily download this data in json format?
Out here in the Western U.S. the Western Regional Climate Center offers such historical data for download. If I recall, their downloads are in .csv format, not .json. However, converting data formats is relatively straightforward for GIS software, such as QGIS.
I don't know how to get it in JSON format, but you can probably write a simple webscraping script for Weather Underground. For example, if you want to make a calendar for your best friend in Greenland, you can use https://www.wunderground.com/history/monthly/gl/kangerlussuaq/BGSF. Or, in HTML terms, https://www.wunderground.com/history/monthly/<country>/<city>/<ICAO>/. Of course, I would recommend going to the Weather underground website and exploring a bit to avoid unexpected behavior.