Maybe you can think of it as a rule where you don't need to make both the subject and the adjective (in fact it's description) plural but just the merely subject which in this case is "Fin" in singular and "Fines" in plural... then just add "de semana" as part of the description of this subject. Another example I can think of is "Estudiante de Postgrado".... that's singular... "Estudiante"... and plurar is "Estudiantes", but you don't say "Estudiantes de postgradoS" the correct form is "Estudiantes de postgrado".

as i have said i had no idea of using this. i'm sorry i didn't reply in time. anyway,thank you for your kind and patient help.

it's not that i don't have the interest in learing. it's that i was not quite familiar with this website at first. I always consider your explanation professional and helpful. I just forgot to selected it as the best answer because i don't know the rules. in fact i'm interested in language.

Muy bien, Luis, siempre contundente!