"Devatenáctá sobota roku je vždy v květnu."

Translation: The nineteenth Saturday of the year is always in May.

[MikhailDidenko]

Why "the year"?! Some specific year or what?

[VladaFu]

'January is the first month of the year'- here 'the' is used in a sense of 'general plus known', as everybody knows what a year is and how many months are there in a year and what they are. https://english.stackexchange.com/questions/370328/whats-the-difference-between-a-year-and-the-year

[Nico44415]

What type of declension is roku in?

[VladaFu]

https://en.wiktionary.org/wiki/rok#Declension

[Nico44415]

So its locative right?

[VladaFu]

It is genitive: "OF the year"

[Annatamoracz]

Why is it "v květnu" here instead of "ve květnu"?

[VladaFu]

For the general rules, see https://forum.duolingo.com/comment/32680720/Mnoho-lid%C3%AD-bydl%C3%AD-ve-m%C4%9Bstech and the Tips and notes (Numbers 2) mentioned there.

However, they do not directly help here. Prepositions before "kv-" will typically not be vocalized. v květnu, v květu, v kvartýru, v kvalitní, v Kvasničkovi, s květnem,...