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