It's all about the little big details that reduce friction in user experience. This attention to detail:
1) reduces errors and user frustration,
2) substantiates the thought in the user's mind that "the software will do what I want", and
3) teaches users that the software will accomodate them, instead of requiring the user to accomodate the software.
It would have been better, actually, if the dates also mentioned the day of week, like "Thursday, October 21 / Friday, October 22". I'm more familiar with what the day of week it is, but not necessarily what the date is.
If it mentioned the weekday, I would be able to answer "Thursday" immediately, since I know that I intended it for Thursday, but I wouldn't necessarily know that it was the 21st without looking at my watch.
If I'm in Siri mode, then I would have to a) turn the phone off and on again to get the lock screen date, or 2) ask Siri, both of which would interrupt this prompt and require you to ask the original question again after finding out what day it is.