This special page takes some text and expands all templates in it recursively. It also expands parser functions like {{#if:...}}, and variables like {{CURRENTDAY}}—in fact pretty much everything in double-braces. It does this by calling the relevant parser stage from MediaWiki itself. One can't supply values for template parameters (e.g. {{{1}}}).
{{#if:...}}
{{CURRENTDAY}}
{{{1}}}