The 6 Nations and RBS announced the end of the sponsorship deal in the middle of last year, with 2017 being the last one.

The new sponsor announcement timing was mentioned back on page 1 of this thread as being the hold up to the release of 2018 fixtures.

From the information earlier in the thread from hospitality websites, it appears the last week of the 2018 competition will be March 17th (St Patrick's Day), although I personally wouldn't book based on this as I have seen mention of one week later on some sites too.