How to calculate first NSDate of current week?

I think this threads responds to what you're looking for: http://www.cocoabuilder.com/archive/cocoa/211648-nsdatecomponents-question.html#211826

Note however that it doesn't handle Mondays as first days of the week, so you may have to tweek it a little by substracting [gregorian firstWeekday] instead of just 1. Also, I modified it to use -currentCalendar, but it's up to you :-)

NSDate *today = [NSDate date];
NSCalendar *gregorian = [NSCalendar currentCalendar];

// Get the weekday component of the current date
NSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit fromDate:today];
/*
Create a date components to represent the number of days to subtract
from the current date.
The weekday value for Sunday in the Gregorian calendar is 1, so
subtract 1 from the number
of days to subtract from the date in question.  (If today's Sunday,
subtract 0 days.)
*/
NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];
/* Substract [gregorian firstWeekday] to handle first day of the week being something else than Sunday */
[componentsToSubtract setDay: - ([weekdayComponents weekday] - [gregorian firstWeekday])];
NSDate *beginningOfWeek = [gregorian dateByAddingComponents:componentsToSubtract toDate:today options:0];

/*
Optional step:
beginningOfWeek now has the same hour, minute, and second as the
original date (today).
To normalize to midnight, extract the year, month, and day components
and create a new date from those components.
*/
NSDateComponents *components = [gregorian components: (NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit)
                                            fromDate: beginningOfWeek];
beginningOfWeek = [gregorian dateFromComponents: components];

The solution by naixn will produce wrong results if monday (2) is set of the firstWeekday and the date you are inspecting is sunday. In this case you will get as a result the monday of next week.

The correct way would be to calculate the subtraction components as follows:

[componentsToSubtract setDay: - ((([weekdayComponents weekday] - [gregorian firstWeekday])
                                  + 7 ) % 7)];

Assuming that you always have 7 days in a week.