Like this?
% Sample Data (Hourly values per day over 10 months)
Time = [datetime(2018,1,1):hours(1):datetime(2018,10,1)]';
Data = rand(size(Time));
YourData = table(Time,Data);
% Convert to timetable
TT = table2timetable(YourData);
% Calculate average value per day
DailyAvg = retime(TT,'daily','mean');