Discourse 3.6.0.beta3-latest (3183a95823).
A new Anniversaries option has appeared in my menu:
I cannot remove it because it does not appear in the list custom list 
AI-generated summary
In a Discourse support thread titled ‘How do I remove the Anniversaries option from the burger menu?’, user Richie reports that the ‘Anniversaries’ menu item appeared unexpectedly in their interface after updating to Discourse 3.6.0.beta3. awesomerobot clarifies that this feature is added by the discourse-cakeday plugin, and can be removed by disabling the plugin — or optionally hidden via CSS. Moin provides a link to a guide on hiding plugin-added sidebar links using CSS. Richie admits they didn’t even know what a ‘cakeday’ was, prompting Lilly to offer to explain. Moin notes that the feature was recently added to Discourse core and is enabled by default, and that all plugins like this will be disabled by default in future updates. Richie expresses surprise, asking for an explanation of why and how it appeared. The discussion centers on removing or hiding the Anniversaries menu item, with solutions ranging from plugin disabling to CSS customization, while also clarifying the origin and purpose of the ‘cakeday’ feature.
That’s added by the discourse-cakeday plugin — disabling the plugin should remove it. Or, do you want to keep the plugin enabled and simply remove the link?
In case you want to hide it with CSS: I wrote a little about hiding links that plugins add to the sidebar in Configuring the default sidebar for your users
I don’t even know what a cakeday is, much less if I want to keep it 
I’ve disabled the plugin, that’s removed it - thanks!
this will help explain 
It was added to core a few weeks ago, and it’s enabled by default 
Ah-ha, that’ll explain how and why it appeared out of nowhere then, thanks