I am looking for volunteering experience that I can highlight on my resume or LinkedIn for an IT help-desk job, but mostly I find ones for a specific day, such as "Volunteers required for Garden Cleaning on this day".
Is it a good idea to mention the date of that one day volunteering for a whole month like "Nov-2016", although it was actually only one day?
Specifying that you volunteered for one event on one day that has no real connection to your job does not really add much to your resume. Even worse, as mentioned in the other answers, lying about the duration or making it unclear would be a very bad move.
However, if there were multiple one day events similar to this that you participated, you can mention something like "volunteered in multiple events of my community, such as ...", which makes it clear that there were short events, but also suggests your implication in the community.
If your date of volunteering was given as Nov-2016 then I would assume that you volunteered for the whole month. If I found out that you actually volunteered for one day only, I would assume that you were intentionally misleading me, that you were totally dishonest, and that you couldn't be trusted. That would greatly reduce your chances of being hired.
If you were hired, and part of the reason for taking you and not someone else was the volunteering, then your job would be in extreme danger if this was found out. You'd be better off not mentioning the volunteering at all. And seriously, if I read "volunteering for one day" I would have questions about your state of mind.
Can I list one day of volunteer work as “Month-Year”?
Well no. Not at all. What you're suggesting isn't just simplifying your resume or cherry-picking what experience to list. You're talking about substantially inflating your experience, i.e. lying on your resume.
Unless specified, listing anything on your resume as "Month - Year" will imply that you spent the majority of that month doing something. If you were volunteering somewhere for a week you'd either list it as "January - Year // Taught week-long training course for children" or, less commonly, "W3 - Year // Taught children's training course".
There is some wiggle room when it comes to rounding off months but that's only when the interval involved is long ehough. So if you officially started on the 31st of July in 2012 it's fine to list "July 2012 - March 2015" on your resume. But doing that if you left after less than a year or two would come across as misrepresenting your experience.
Lying on your resume is such an immense breach of trust that if it was discovered in the interview process you'd be immediately dropped from consideration. If it was found out after you started the job you'd risk being fired for cause in many companies but even in companies that don't immediately go that far it would be a giant black mark on your record and reputation. The only time where this wouldn't have such a dramatic impact was if it was just an honest mistake or a typo, but that would typically be discovered in an interview. If you actually lied about it during an interview, you'll suddenly have plenty of time to write a new resume.
Why indicate the date? You could simply indicate the action you took voluntier avoiding that issue.
Is it a good idea to mention the date of that one day volunteering for a whole month like "Nov-2016", although it was actually only one day?
There's absolutely no need to mention any dates in volunteer activities.
Normally, you'd list your volunteer work under a heading of "Volunteer Work" or such. Just list the activities with no dates attached.
The duration of some will be self-evident
Others will just be non-specific
You seem to be intending to deceive the reader into thinking your single-day volunteer work was actually more than that. You would be embarrassed if you are asked about it and only then have to admit it was only a day. Better not to put yourself in that position.
