Buses replace trains between Manchester Victoria and Stalybridge for the next 5 Sundays (6th April to 4th May inclusive).
A revised timetable will be in operation with trains running between Huddersfield and Stalybridge only.
http://www.northernrail.org/pdfs/trackworks/0102a_Huddersfield-Manchester.pdf
Although the timetable shows connecting buses at Stalybridge, it may be quicker and more convenient to change on to an express to Piccadilly (5 minute connection towards Manchester, 3 to 5 minute connection coming from Manchester).
Definitely avoid the bus on the afternoon of Sunday 19th April, because Manchester City are playing at home on that day.