: Having the main page include [[April 19]] (that is, as a new template, not as [[April 19]]), however, would mean having to create a whole series of nested templates, involving changing every single link to a date in the whole wiki (many thousands of links).
: [[April 19]] is not a template and cannot be included as one. So, to keep the date links simple throughout the wiki (i.e., linking simply to the date and not to a template), [[April 19]] would need to include a [[Template:April 19]], which would in turn include nested templates for [[Template:April 19 (new)]] and [[Template:April 19 (old)]].
: But that then introduces confusion for folks who click on [[April 19]] and see two sets of feasts, one labeled as "Old" and one as "New." This would be a problem, for instance, in the [[Annunciation]] article. You click on [[March 25]] and see that the "New" feasts include Annunciation, but the "Old" do not. Does that mean that Old Calendarists aren't celebrating the Annunciation on March 25? In fact, they ''are'' celebrating it on March 25, but by another calendar. I can think of no way of solving that problem.