Why five day and not five days?

Question

Why here is "five day": I follow the Effortless English five day training method.

and why here is "five days": I practice every unit for at least five days.

Answer 1

I follow the Effortless English five day training method. = "five-day training method" here is the single event "five-day training method". Five day = here is an adverb.

I practice every unit for at least five days. = "five days" here are "at least five" separate events, how many days.

Answer 2

Under Stalin, the Soviet Union had "five year plans", or at any event that is how the name of the plans was translated into English. If, instead, there had been a new plan every year then after five years there would have been five yearly plans.