dnia

Dnia is the genitive of dzień, day, and opens most dates: dnia piątego maja, on the fifth day of May. The day number is an ordinal in the genitive, ending in -ego (pierwszego, drugiego, piątego), and the month that follows is also in the genitive.

Some records give two dates, as dnia piątego/siedemnastego maja: the first is Julian (used by the Russian Empire) and the second Gregorian, twelve days later in the 1800s. Dnia wczorajszego means yesterday and w dniu dzisiejszym means today, counted from the date of the entry.