How long after written notice does tenant have to move out?

[Sophia Remley]

Hello all! I am a newbie out of Winchester, VA and am closing on my first property in WV in 2 weeks. It is a Duplex and both sides are currently rented month to month with 30 day notice. I am buying this as a primary residence so the plan is to give notice to the tenant on the side that needs the most work. My original closing date was today and I had planned on giving written notice to the tenant immediately and they would have to move out by 12/31

My closing date was pushed back to 12/3/2020 because I couldn't get an appraisal scheduled because of all the refinances. My question is, if I give written notice to the tenant on 12/3 does that mean that the tenant has until 1/2/2020, 30 days later to move out? Or will they have until the end of the month? 

I'm probably overthinking this but I want to get it right the first time!