We use "vacanze estive" to express the extension of a period (in terms of months or weeks, therefore longer than a weekend) in which we interrupted our working or studying activities. 

PLURAL:

Ex: Durante le vacanze estive sono andato al mare. (long period: could be 15 days, 1/2 months...)

SINGULAR:

Ex: Questo fine settimana sarò in VACANZA (short period, just the weekend, and not everyone will interrupt their working activity, is YOUR decision, not institutionalized) OR Ex2: Mi prendo tre giorni di vacanza

Similarly for Christmas:

Ex: Durante le vacanze natalizie (or di Natale) andrò a sciare. (a LARGE part of the Christmas holiday will be spend by going skiing)

Anyway, I guess that the plural expression is partially due to the fact that schools and some other institutions close in that periods, so the summer holiday is not just the mine, but also the summer holiday of schools, universities, institutional offices and buildings... "le vacanze estive" are something common and shared by all :)

morale : si usano tutti e due e a seconda del riferimento al verbo o ai tempi cambia! Con la pratica cu arriverai! Non mi risulta sia un idioma! Ciao