Putin's annexation ceremony will be held in one of the Kremlin's grandest halls with the pro-Russian figures Moscow considers to be leaders of the four Ukrainian regions. Russia says the referendums were genuine and showed public support for the move.
Putin's spokesman Dmitry Peskov said agreements will be signed "with all four territories that held referendums" and Putin would deliver a major speech.
A big concert will be held on Friday on Moscow's Red Square, where a tribune with giant video screens has already been set up, with billboards proclaiming "Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!"
Peskov did not say whether Putin would make an appearance at the concert. He did so at a similar event in 2014 after Russia proclaimed it had annexed Ukraine's Crimea region.