I had a moment of panic last night when most days in the past year reported no posts. I initially thought that stuff had gotten deleted or the database corrupted. That wouldn't have been too much of an issue thanks to nightly backups but the panic was still real.
On This Day was reporting posts for each of the days so I realised the issue was that I had forgotten to rewrite some of the database queries after reworking scheduled posts. A few edits later and normality (and my sanity) was restored.