Calendar hack: problem now with php the_date

First I don’t know if I’m right to post a new topic but I feel like this problem although directly related to the following post is maybe somehow very specific to my WP usage.
Shortly, I wanted to have the calendar in Finnish and starting with Monday. Through the instructions given on those threads –
1- https://www.ads-software.com/support/3/1511
2- https://www.ads-software.com/support/3/2629
– I ended up after modifying the $weekday[0]=’Sunday’ to ‘Monday’ in wp-config-extra.php to have a calendar displaying everything correctly.
Great! And thanks to all great contributors I was very happy to have this done.
Well, I’m facing now another problem which is obviously connected to the change of $weekday[0]=’Monday’.
As a result, calling php the_date in my index.php with the following settings, l, d/m/Y returns the day (l) wrong, showing Saturday instead of Friday.
My php knowledge is very tight (willing to learn though!) and I’ve tried to make changes but I’ve basically no idea on what I’m really doing and I don’t feel like messing up (again!) with template-function.php or any other file.
I believe the calendar hack is important for many end users as in a bunch of countries starting a week with Monday makes more sense than with Sunday.
Could someone help me to figure out how to keep my calendar looking like I want and at the same time being able to display the day in full characters in my <h2> headings?
Hope I’m not asking too much!
Thanks for helping me…