Randy is right that the apostrophe is unnecessary. However, the meanings are not necessarily the same. I may get three weeks of holiday in a year, and may (for example) take one week of holiday every four months. But a three-week holiday means I take one holiday that lasts for three weeks (i.e., 21 days).

There are both understandable, with the same meaning, but the first is poor English. Do not give the three weeks possession of the holiday. The apostrophe is a mark best avoided, as much as possible.

apostrophe to indicate possession, I thought,.. for example: I borrowed rani's book yesterday i went to Rissa's Home