This is the 254th most frequent Slovak word.
celý
Whole; entire.
Here, 'celý' functions as an adjective describing the noun 'deň,' indicating the entire extent of the duration.
Strávil som celý deň prácou.
I spent the whole day working.
'Celú' is the accusative singular feminine form of 'celý,' modifying the noun 'tortu' to stress that it was the entire cake eaten.
Jedla celú tortu sama.
She ate the whole cake herself.
In this context, 'celý' intensifies the adverb 'zle,' emphasizing the absolute nature of the poor planning.