Pasting files and concatating files together

I have various years and months of data for numerous site locations. I have computed the average for all months for each needed year. Each Month/Year has its own its own file. Not all sites have data for a given month so no file exists in that case. What I am attempting to do is take each file Month/Year to build a new file for each site. The file structure I am attempting to build would be |Year | Jan| Feb| |:----|:---|:---| |2002 | xxx| xxx| |2003 | xxx|xxx | |2019 |xxx |xxx | the naming conventions of the files are County071-O3-5001-**2002**.out-**APR**.csv.tmp.ext-avg.dat with the **Bold** text which would change depending on the year and the month. Since there are missing "Months" and Years" data at times I am unsure how to build table to account for missing months of data. Any suggestions how to automate this would be great given I have over 100 sites to do for a 17 year period of time. Thanks