Did you know that to tell the date we use the Nominative case of an ordinal number, and to tell the date of an event or an action we use the Genitive case of an ordinal number?
Months and years are used in the Genitive case.
Сегодня пятое марта.
It is 5th of March today (Nominative).
Пятого марта мы были в ресторане.
We were in a restaurant on the 5th of March (Genitive).