score:14
Your intuition is right, it is before. The minus indicates that you should substract. If you wanted to add you would use a plus.
If today (2021-12-24) was for some reason important to you, today would be the day J (le jour J in French). J-2 would be 2021-12-22 and J+2 would be 2021-12-26. The use of minus is much more common than the use of plus.
So to specify a bit regarding your question, it doesn't specify a period but a specific day. From the context I deduce that the day J is your arrival date at the hotel.