When does the Northern Hemisphere reach minimum land area covered by solid H2O?
A little Google searching and I found this:



Source:  https://nsidc.org/cryosphere/sotc/snow_extent.html

Snow-cover is considered an important factor in climate change study, so it's tracked pretty carefully.   There's probably a specific average minimum date out there somewhere if you're really interested but I didn't see it.  Judging from the chart, somewhere around mid August.   

Sea ice cover is later, that reaches a minimum in September most years, but oceans are slower to change than land and air, so that should be expected.   The hottest day on average, if you care about that, is July 24.   http://m.weatherbug.com/weather-news/weather-reports/5225