Learn Russian: Мы празднуем годовщину десятого августа две тысячи двадцатого года. - We are celebrating the anniversary on the tenth of August two thousand twenty.

To say that an event happens on a specific full date, Russian puts the ordinal number for the day into the genitive case. So, instead of the nominative десятое, we use the genitive десятого (literally "of the tenth"). The month (августа) is also in the genitive, meaning "of August".

Why are the words for "two thousand" (две тысячи) not in the genitive case like the rest of the date?

When expressing a year in Russian, only the very last word becomes an ordinal number (like "twentieth") and changes its case to match the grammar of the sentence. The earlier parts of the number stay as standard, unchanging base numbers. That's why две тысячи (two thousand) doesn't change, while the final word becomes the genitive двадцатого.

The infinitive for "celebrate" is праздновать. Where does the -у- in празднуем come from?

This is a common, tricky rule for verbs ending in -овать. In the present tense, the -ова- part drops out entirely and is replaced by -у- before you add the standard endings. That is why праздновать becomes мы празднуем (we celebrate) instead of мы праздноваем.

Why is the word for anniversary годовщину here instead of годовщина?