Try this:
\define getCounts()
<$list filter="[!is[system]eachday[created]get[created]]">
{{{ [sameday<currentTiddler>count[]pad[3]] }}};<<currentTiddler>><br>
</$list>
\end
<$wikify name="countlist" text=<<getCounts>>>
On {{{ [enlist<countlist>sort[]last[]split[;]rest[]] }}} {{{ [enlist<countlist>sort[]last[]split[;]first[]trim[0]] }}} tiddlers were created<br>
</$wikify>
Notes:
1) Just like your in original code, the getCounts() macro produces a list of each day and the number of tiddlers created for that day
2) However, the output is formatted as "count;date", where the count is zero-padded to 3 digits so all count values are the same number of characters
3) The <<getCounts>> macro is passed to $wikify to capture its output in a variable named "countlist"
4) Then, using filtered transclusion, enlist[] the captured output, sort[] it in ascending order (i.e., by the 3 digit "count" value) and get the last item (i.e., the one with the largest count value)
5) To produce the date output, you split[] the last item at the ";" separator and use rest[] to get the date
6) To produce the count output, you split[] the last item at the ";" separator and use first[] to get the count, trimmed to remove any zero padding
enjoy,
-e