If anyone in the future is looking for an anser: I wrked it out myself. Its definitely not the most elegant solution, but it works so good enough for me
%% sum each 5 days from 1-Jan-2015
Aggregate_5day_1 = retime(Aggregate_1day, 'regular', 'sum', 'TimeStep', caldays(5));
% find annual max of these values
Aggregate_5day_1_max = retime(Aggregate_5day_1,"yearly", "max");
%% sum each 5 days from 2-Jan-2015
% delete 1-Jan-2015 from Aggregate_1day timetable
Aggregate_1day(1,:) = [];
Aggregate_5day_2 = retime(Aggregate_1day, 'regular', 'sum', 'TimeStep', caldays(5));
% find annual max of these values
Aggregate_5day_2_max = retime(Aggregate_5day_2,"yearly", "max");
%% sum each 5 days from 3-Jan-2015
% delete 2-Jan-2015 from Aggregate_1day timetable
Aggregate_1day(1,:) = [];
Aggregate_5day_3 = retime(Aggregate_1day, 'regular', 'sum', 'TimeStep', caldays(5));
% find annual max of these values
Aggregate_5day_3_max = retime(Aggregate_5day_3,"yearly", "max");
%% sum each 5 days from 4-Jan-2015
% delete 3-Jan-2015 from Aggregate_1day timetable
Aggregate_1day(1,:) = [];
Aggregate_5day_4 = retime(Aggregate_1day, 'regular', 'sum', 'TimeStep', caldays(5));
% find annual max of these values
Aggregate_5day_4_max = retime(Aggregate_5day_4,"yearly", "max");
%% sum each 5 days from 5-Jan-2015
% delete 4-Jan-2015 from Aggregate_1day timetable
Aggregate_1day(1,:) = [];
Aggregate_5day_5 = retime(Aggregate_1day, 'regular', 'sum', 'TimeStep', caldays(5));
% find annual max of these values
Aggregate_5day_5_max = retime(Aggregate_5day_5,"yearly", "max");
%% Find annual total max from the max of those 5 values for each year
Aggregate_5day_temp = synchronize(Aggregate_5day_1_max, Aggregate_5day_2_max, Aggregate_5day_3_max, Aggregate_5day_4_max, Aggregate_5day_5_max);
Wet3_7_5day = timetable(Aggregate_5day_temp.Time,Aggregate_5day_temp.Wet3_7_Aggregate_5day_1_max, Aggregate_5day_temp.Wet3_7_Aggregate_5day_2_max, ...
Aggregate_5day_temp.Wet3_7_Aggregate_5day_3_max, Aggregate_5day_temp.Wet3_7_Aggregate_5day_4_max, Aggregate_5day_temp.Wet3_7_Aggregate_5day_5_max);
Wet3_7_5day_max_temp = max([Wet3_7_5day.Var1, Wet3_7_5day.Var2, Wet3_7_5day.Var3, Wet3_7_5day.Var4, Wet3_7_5day.Var5], [], 2);
Wet3_7_5day_max = array2timetable(Wet3_7_5day_max_temp, "RowTimes", Wet3_7_5day.Time);