This is the 1288th most frequent Polish word.
kolejnego
"Kolejnego" means "next" or "another" in English, depending on context.
Here 'kolejnego' is used as an adjective in the genitive case to describe 'deseru', meaning 'another'. It signifies a sequence rather than merely doubling.
Zjadłem pełny obiad i zamówiłem kolejnego deseru.
I ate a full lunch and ordered another dessert.
In this context, 'kolejnego' modifies 'dnia' in the genitive case, signifying the upcoming instance in a calendar.
Po skończonej pracy zaczęliśmy planowanie kolejnego dnia.
After finishing work, we started planning the next day.
Here 'kolejnego' refers to an item in a sequence, the next train, modifying 'pociągu' also in the genitive case.