I need your help to do this, I 've been trying it for a couple hours unsuccessfully

Every day I receive some files like this:
209_077_131223230450.txt
102_077_131223230450.txt
019_077_131223230451.txt
319_077_131223230451.txt
209_077_131222230450.txt
102_077_131222230450.txt
019_077_131222230451.txt
319_077_131222230451.txt
You can notice that the first six characters after _ are the date (2013-12-23).
Well I want to create a program that concatenate all the files of the same date.
What I'm doing by now is writing this code:
copy *131222*.txt 131222.txt
But I d like something more automatic, something like
for %%date in (!each date that appear in my directory) do (copy *%%date*.txt %%date.txt)
But I don't know how to do it

Thanks for your help. If you need more information, please let me know.