The “weather” of a given Debian-based distribution is an indication of how safe it is on a given day to attempt some package installation/upgrade.
Check the weather here
A “bad day” is a day in which a sensible percentage of that distribution repository is not installable due to unsatisfiable inter-package dependencies. A “good day”, on the contrary, is when most (possibly all) of the packages available in that distribution repository are installable. For more information see the thresholds below.