Administration: Always display the On This Day dashboard widget

[i-am-chitti]

Trac ticket

https://core.trac.wordpress.org/ticket/65647

Problem

The On This Day dashboard widget is hidden with the core hidden class when there are no posts to show for the current calendar day. This creates inconsistent, confusing behavior:

• The widget is not visible on the dashboard, but its Screen Options checkbox is still checked — so the reported state disagrees with what's actually shown.
• Unchecking and re-checking the option reveals the widget, because postbox.js calls jQuery's .show(), which injects an inline display: block that overrides the .hidden class.

The root cause is two independent hide mechanisms competing: a server-side hidden class vs. the persisted per-user Screen Options preference.

Fix

Take the "always show" approach (option 1 from the ticket):

• Remove the postbox_classes_dashboard_wp_dashboard_on_this_day filter and the wp_dashboard_on_this_day_postbox_classes() function that added the hidden class.
• The widget is now always registered and always visible. When there are no matching posts, the existing placeholder — "No posts were published on this day in previous years." — is shown.
• Hiding is deferred entirely to Screen Options, which persists correctly and keeps the checkbox state consistent with on-screen visibility.

This is consistent with other always-registered dashboard widgets (e.g. Activity, At a Glance) and avoids !important CSS hacks.

Testing instructions

1. Ensure there are no published posts dated on today's month/day in a previous year (a recent install satisfies this by default).
2. Visit the Dashboard.
3. Before this change: the widget is not visible, yet "On This Day" is checked in Screen Options; unchecking and re-checking reveals it.
4. After this change: the widget is always visible and shows the placeholder message; the Screen Options checkbox state matches its visibility. Unchecking hides it and the preference persists across reloads.

Screenshots

Before

After

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Opus 4.8
Used for: update unit tests

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[mukeshpanchal27]

Thanks @i-am-chitti for the PR!

Overall look solid to me.