Crimea returned as a Russian federal region, after a referendum that took place on March 16, 2014, in Crimea and Sevastopol, and both regions became within the Russian Federation, as of March 18, 2014, and this day was considered an official holiday in Crimea, and the city Sevastopol.
Summary
Crimea returned to Russia after a referendum in March 2014, in which the vast majority of the electorate in the Republic of Crimea and Sevastopol voted to join Russia.
Crimean authorities held a referendum in the wake of a coup in Ukraine in February 2014. Ukraine still considers Crimea to be its own.
The Russian leadership has repeatedly stated that the people of Crimea voted in a democratic way for reunification with Russia within the framework of international laws and the United Nations Charter.
Disproof
Recurring pro-Kremlin disinformation narrative about the illegal annexation of Crimea. No international body recognises the so-called referendum, announced 27th of February and held less than three weeks later.
On March 27th, 2014, the UN General Assembly adopted a resolution in which it stated that the referendum in Crimea was not valid and could not serve as a basis for any change in the status of the peninsula.
The oft-cited figure of 97% has been contested by the Kremlin’s own Human Rights Council, which estimated that only between 30% and 50% of Crimeans took part in the referendum, of which some 50-60% favoured secession.
See here for the EU statement on the sixth anniversary of Crimea annexation. On 10 September 2020 EU extended "the sanctions targeting persons and entities that continue to undermine or threaten the territorial integrity, sovereignty and independence of Ukraine."