The electric lines (slow lines for those that don't speak Great Eastern) will be closed from Christmas 2016 to Mayday bank holiday 2017 between Brentwood and Shenfield. This is to enable the massive task of remodelling Shenfield to take place, but still only closing the main lines between Christmas and New Year.
Monday to Friday the timetable is as indicated, with buses between Brentwood and Shenfield (except the first up and last down train which run on the main lines). GA trains do not call at Brentwood, simply because it can't be timetabled (or not without a huge disproportionate effect). Plus peak 12 cars couldn't call anyway, so the simple solution is a frequent bus service for one stop.
Weekends are a different issue. Due to other work elsewhere, the TfL service can only get to Brentwood on relatively few occasions. There is so much going on that the weekend timetable will be amended in all sorts of ways. Therefore there is little point in producing a permanent weekend timetable that only amends the Shenfield part of the work, only to have to further amend it to account for everything else, much of which is different each week. There is small print in the top right of the timetable warning of weekend engineering work.
Stopping weekend GA services at Brentwood on the up is relatively easy, but the added time on the down starts to cause ripples across East Anglia, especially with connections onto single line branches, unless the trains leave Liverpool St earlier. You then start to affect everyone, just to assist the relatively few making intermediate journeys between Gidea Park, Harold Wood or Brentwood to / from beyond Shenfield. Mind you those GA services won't get as far as Brentwood many weekends anyway.
Details of weekend timetables should be available at the usual 12 weeks beforehand, but the volume of change makes that quite a challenge for everyone involved. Hope that answers a few questions.