Automatically running a macro

So i have a couple hundred files every month that need to have a header added to them after the analyst completes them. The macro is already written and i have it set up with a short cut so its not a terrible process, but i can't help but think there has to be a way to set it up so that I can select all the excel files at once and just open them all and have the macro run automatically without having to press any buttons which will save a lot of time over the course of a year. All the files that need to have the macro run are in the same folder so i was just wondering if there is a way to edit the macro so that it will run automatically upon opening the files in only the folder that contains them. I am proficient with excel, but not the macro side of it so if you could put it in lamen's terms that would be excellent. If you have the time a step by step would be very helpful. Again the macro is already created and saved on my computer i just want to edit it to make it more efficient so i don't manually have to run it on every single workbook.