If you wanted to all the sundays into mondays, it would be easy: combinecats. But you want to turn them into either mondays or fridays depending on whether they are followed by a monday or a tuesday. Right?
You should be able to do this without loops:
i1 = (t.td(1:end-1) == 'Sunday') && (t.td(2:end) == 'Monday');
t.td(i1) = 'Friday';
i2 = (t.td(1:end-1) == 'Sunday') && (t.td(2:end) == 'Tuesday');
t.td(i2) = 'Monday';
t.td = removecats(t.td); % drop the now unused Sunday