On second thought, maybe this would sound better as, %s set this event to repeat %s?

The other date-related transaction feed stories don't actually specify the new value, so let's drop it here for consistency. There also might be some peskiness related to formatting and rendering the date, and because we don't actually ever hit this case, we probably want to avoid any uncertainties.

Also, this isn't the "end date" of the event (that's something separate). This is the "recurrence end date".

This should match what is in getTitle as best as it can. So, %s made %s a recurring event.

For now, this only shows up on event creation, and there is no "change" in that case. Would be more versatile to make this, %s set %s to repeat %s

We already have a separate transaction for TYPE_END_DATE and this could be confusing, so we should definitely mention the "recurrence" part. And, as in a previous comment, we don't actually include the new date in other date-related transactions, so let's drop it.

"of *this* event"

Actually, getTitleForFeed does add the new value to the story, so it would make sense to add it here. But, again, "changed" seems inaccurate, as this is on event creation.

For consistency, just say "%s made this event recurring." instead of "%s has made this event recurring." (that is, no "has"). We don't use the word "has" in other strings (for example: "%s created this revision.", not "%s has created this revision.").

This isn't translatable because the second parameter will be an English word like "daily", "weekly", or "monthly".

It's also not sufficient to just translate the word fragments separately. In some languages, the rest of the sentence may depend on the word. For example, it's possible that this phrase translates into Spanish most naturally like "repeats on the x", as either "..repetere un la dia." or "...repetere un el mes.", where the use of the article "la" or "el" varies depending on whether the noun "day" or "month" is feminine or masculine (I don't know Spanish and this specific case is probably not correct, but this is a general issue with translating into other languages).

To allow translators to translate this string properly, each variant of it needs to be separated out, something like this:

switch ($new) {
  case SomeCalendarClass::SOME_DAILY_CONSTANT:
    return pht('%s set this event to repeat daily.', ...);
  case SomeCalendarClass::SOME_MONTHLY_CONSTANT:
    return pht('%s set this event to repeat monthly.', ...);
  ...
}

Then foreign language translators can translate the word and adjust articles appropriately.

Drop "has" here, too.

Same "daily" issue here.