(Look up 'wagner sledgehammer' and 'Bucha' if you want to understand and be sickened). As the russian army likes to send human wave attacks to flush out Ukrainian positions and then leave their troops to die (or buries them alive in mass graves), attack civilian infrastructure (hospitals, schools, playgrounds), torture and rape prisoners, and generally revel in their war crimes, many of us (but not all) think it is a fitting name for the invading soldiers committing atrocities some consider it a backronym for 'occupying russian criminals'. It references 'The Last Ringbearer' where russia is Mordor invading the Ukrainian elves. The Ukrainians started calling them this, and some russians are proud of the name. Kiev was the russian name, Kyiv is the current Ukranian name. Officially, the country's name is Ukraine, not 'The Ukraine'. Well, you can, but it's considered an insult, because it's what Russia called them ('the borderlands'?), and suggests they're not a real country. For 2023, there was a contest to see who could land a Ukrainian colored drone in Red Square.
They usually have a big parade in moscow to celebrate. The date that russia recognizes as the day that they 'beat' Germany in 'The Great Patriotic War' (aka World War 2, where they lost 20 million soldiers). For those new to these threads, a mini FAQ: