As always with these questions, if you're uncertain what to answer, give the answer that results in tbe greater disclosure of information.
By "these questions," I mean questions designed to elicit information that would make you ineligible for naturalization, where your related history does not make you ineligible for naturalization.
Consider: there are two possible answers, yes or no, and two possible opinions of USCIS as to whether the Defense Preparation Day requires you to answer yes. If you answer as they think you ought to answer, there is obviously no problem. There are two possible scenarios where your answer is the opposite of the answer they would expect from someone in your circumstances:
If you answer no, and they think you should have answered yes, then you expose yourself to a finding of deception and possible removal from the United States.
If you answer yes, and they don't think you needed to answer yes, then you've given them more information than they needed, which they can disregard, and all is well.
The only scenario that exposes you to potential trouble is one in which you answer "no" but USCIS finds it deceptive. There is no scenario in which answering "yes" would lead to undesired consequences. Therefore, you should answer "yes."