Each time through the loop data contains the data from the current file, so that when you do save, only the current file's data will be saved in the mat-file, overwriting what was in the mat-file before (if anything). If you want one mat-file with the data from all the text files, you can accumulate the data in the loop and then save it once after the loop.
Files_September = dir('202109*.txt')
files = numel(Files_September);
data = '';
for k = 1:files
files2 = Files_September(k).name;
fid = fopen(files2);
new_data = fscanf(fid, '%c');
fclose(fid);
data = [data new_data];
end
save('September','data')
Now data will be a character array of all the text in all the files and it will be stored in the file 'September.mat'. Maybe this is sufficient and maybe not; I don't know what kind of data is in those text files. Each file contains three columns, but three columns of what? Numbers? Names of countries? And are there line(s) of text at the top with column titles? The answers to those types of questions would determine which method is best for reading the files and accumulating the data properly.
Can you attach an example text file to your question so that we may help further? Thanks.
