Fixed datum – help with script

Hello.
I am creating reservation system for a client and she wants to set one fixed date 20 dec – 26 dec, this date should be possible to reserve only together. That means that if I click any date from 20 – 26 dec the whole range will be chosen.
I am working on a script. Would you be so kind and help me point me in the right direction?What function to call? It should work – if I click some date, choose also another date. What is the function for selecting dates and how to call it? thanks