Timesheet screen does not depend on off days.
I always start on monday...

You always start on Monday because it is the first working day in Europe and USA , In the middeleast the first day working day is Sunday and some places it is Saturday , Can enhancement be added to this area allowing you to choose the first day appearing in the week of time sheet

Hi,

The request already exists in the roadmap (#2704) with low priority (high cost, low benefit)

Hello,

I'm looking to modify the code myself to implement that function since it will be easier for our organization.

Could you point me out where to start looking since projeqtor is a big project.

Regards

Unfortunately it(s not an easy piece of cake.
There are many places where "firt day of week" is important.

Maybe you can have a look at function

function firstDayofWeek($week=null, $year=null) {

because this, at least, will have to be changed.

But it's not the only one. Have a look at

function lastDayofWeek($week=null, $year=null) {
function weekNumber($dateValue) {
function weekFormat($dateValue) {
function getWeekNumberFromDate($date) {

and possibly other functions (at least all thoose who deal with week)

Andd there you'll have to investagate everywhere thoose functions are called to check how their result is used...