The "Placed is service date" according to the IRS is when you made the property ready and available for rent.
For a short-term rental this could be the date that you listed it on Airbnb or VRBO and not necessarily when your first tenant / guest stayed in the property. Alternatively, for long-term rentals this can be the date that you started looking for tenants and the place was ready to be occupied.
This date is important because it determines when you can start depreciating the building and also related assets like furniture and fixtures.
More here from the IRS on how the placed in service date works and depreciation
This article is provided for illustrative purposes only; it does not provide personalized tax, legal, financial, or other professional advice. Your situation may be different; consult a professional for information concerning your individual tax, financial, or legal situation before taking any action.