According to Launching Films:

 

From what I can gather about “Venue:  saturation”,  it appears that:

A saturation release ‘at cinemas everywhere’ may open simultaneously on 1,000 screens UK-wide, playing at two or more screens per multiplex. This strategy, usually deployed for ‘tentpole’ titles such as large-scale sequels or star-led holiday releases, helps to accommodate mass audiences eager to consume a film at the earliest opportunity.

You can compare releases:  La La Land, Assassin’s Creed, Lion & Jackie, for example, are listed as “Venue: saturation”.  Meanwhile Manchester by the Sea is “Venue: wide”. I have to say if I was in the UK I would be ecstatic that I’m not seeing the word “Key Cities” or “Limited”.

Remember though, all release dates are subject to change, but at this stage it looks like 24 March 2017 is definitely the date.