We have a matrix containing the year month and day of data points. We want to find out how often each day occurs. Thanks in advance!

We want to know how often a day repeats.

Answer 1

[Isabella Osetinsky-Tzidaki]

% A is N by 3 matrix (y,M,d)

B=datetime(A,'Format','yyyyMMdd');

C=char(B);

D=str2num(C);

F=unique(D);

L=length(F);

Answer=nan(L,2); % pre-allocate date and its occurrence columns

for i=1:L

    n=length(find(D==F(i)));

    Answer(i,:)=[F(i) n];

end

Answer 2

[Star Strider]

Another approach:

M = [2016 12 12;  2016 12 11;  2016 12 11;  2016 12 10];    % Create Data
dn = datenum( M );                                          % Compute Date Numbers
[Ud,ia,ic] = unique(dn, 'stable');                          % The 'stable' Argument Retains The Original Order
date_tally = accumarray(ic, 1);                             % Count Occurrences
Result = [M(ia,:), date_tally]

Result =
        2016          12          12           1
        2016          12          11           2
        2016          12          10           1

You will need to make appropriate changes for the format of your dates (you did not specify what they were). This should do what you want.