Pre-making pastilla filling
I'm in the US and when we buy fresh chicken at the market it has a 'sell by' date. It's safe to assume that it will be good for one to two days after that date, as long as the chicken is stored properly. If this is the case for you, try and get your chicken with a sell by date of at least Saturday. Then you can be assured it will be okay on Sunday or Monday when you make your filling. Just be sure to get it home and in the refrigerator right away.

If this is not an option for you, there are a couple of things you can do: 


Assuming you buy your chicken on Thursday, take it home and put it in the freezer right away. On the day before you wish to make your filling, place it in the refrigerator to thaw overnight. Then proceed as usual. A very short time frozen should not affect the quality or taste of your meat.
You can make your filling in advance and refrigerate it. After cooking, chicken should be okay in the refrigerator for three to four days. So, you can make your filling on Friday or Saturday and refrigerate it until you bake your pastilla on Monday. I wouldn't think this would hurt the flavor in any way. Many times the overall flavor can actually improve as the flavors have time to meld together.