Date facet drilldown
This module enables yearly date facets to drill down to monthly granularity, preserving "pretty paths" for cleaner URLs. It's designed to be simple, requiring only a checkbox to be enabled on your year facet configuration. This functionality works specifically with "AND" type facets.
This module is built for one purpose only - to allow yearly date facets to drill-down to month level.
(This functionality was available in Drupal 7 but has not yet been provided in Drupal9.)
The impressive facet_granular_date module does implement this functionality but unfortunately does not work with facet 'pretty paths'.
The idea of this module is to be as simple as possible - there is no need to change the source of your facets, you can just leave the URL processor as 'pretty paths' (the latest 3.x release will also work with 'Query Strings').
To use this module, just install it and then edit the facet that you wish to use it for. You should use the 'Date item processor' and set the granularity to 'year'. You will see a new checkbox labelled 'Date facet drilldown' so just check this and the drilldown should be available.
Note that this module only works for 'AND' facets and not for 'OR'. This is because it wouldn't make any sense to select a year facet and then select a month within it with an 'or' condition, that only works when we select data within a year and within a month.
The latest release 8.x-3.x-dev now works with Facets 2.x and is the recommended release even if you are still using the older Facets 1.x releases.
Note that if you are using Facets 2.x and pretty paths, you must install Guy Schneerson's excellent patch https://www.drupal.org/files/issues/2023-01-12/url_duplication-3254600-4... which is mentioned in this issue https://www.drupal.org/project/facets_pretty_paths/issues/3254600#commen...