Book that i had read in highschool in 2018, about 4-6 Adults who came to a wooden house near a forest for the summer holiday(not sexual).

[Diaconescu]

One of them used to speak at the phone every day, another one to keep sending letters to a friend from the city, another one in the mornings was writing on a blackboard the wheather humidity, wind force and temperature. All of them where not fully disconnected from their lifes(to enjoy the holiday) so ONE adult starts sabotaging them one by one until no communication possibility was left to communicate with the outside world...so eventually they start spending time together, not knowing what time or day it is..."it was just a day, like others ".
At the end of the holiday they start checking in which day they are and what time it is, because they realise that the holiday it s almost over.

[Capn]

Do you remember anything more about the setting? A specific country, or even a state/province, or the city the friend was writing letters to?

Did you read it in English, or another language? And which country were you in when you read it? :)

Can you remember any names of any of the characters? (Was it a cell phone they used, or a payphone or landline?)