The easiest and cleanest way uses the $transclude widget with the index attribute to get a property from a data tiddler:

It's ''<$transclude tiddler="UsHolidays" index=<<now "0MM/0DD">> />'' today!

However, we haven't discussed the index attribute of the $transclude widget, so it's unlikely you thought of that unless you assumed it must exist and visited the documentation (but kudos if you did, that's the right way to think about widgets)! Here's a way to accomplish this using only tools we've already seen:

<$let formattedToday=<<now "0MM/0DD">>>
  It's ''<$text text={{{ [[UsHolidays]getindex<formattedToday>] }}} />'' today!
</$let>

(Recall that the text widget is needed to prevent the holiday from turning into a link with the use of triple curly braces.)