Hi Michelle. Since I do not know the exact structure of your data, I'll provide a few pointers which you can use to solve your problem. I'm assuming that every file is named "northward_winds_Np.YYYYMMDD.nc4" and that the matrix has a separate column indicating whether the event of interest occured on that date or not.
To get the list of files on which the event occured, the following "for" loop will work. The matrix has been named "data" and the first, second and third column are assumed to contain the year, month and day values respectively.
year = num2str(data(i,1));
month = strcat("0",num2str(data(i,2)));
month = num2str(data(i,2));
day = strcat("0",num2str(data(i,3)));
day = num2str(data(i,3));
file_name = strcat("northward_winds_Np.",year,month,day,".nc4");
relevant_files = [relevant_files; file_name];
Once you have the list of all the files you need, you can use the "copyfile" function to copy these files to a different folder.