My dear friend Sibab, we will consider the month of March 2014, which has 31 days and 21 working days.
March 2014 Attendance Scenario
If the offer letter defines the 3rd day of the month as the joining date, the calendar month's 1st working day starts on Monday, 03.03.2014. The total working days are 21. If Mr. A joins on 03.03.2014, he starts his new job from the 1st working day of the month and successfully attends all 21 working days (100% attendance). He did not miss attending any working day, so he is eligible for full salary.
April 2014 Attendance Scenario
We will consider the month of April 2014, which has 30 days and 22 working days.
If the offer letter defines the 3rd day of the month as the joining date, the calendar month's 1st working day starts on Tuesday, 01.04.2014. The total working days are 22. If Mr. A joins on 03.04.2014, he starts his new job from the 3rd working day of the month. He misses the first 2 working days of the month, so he attends only 20 working days (approximately 91% attendance) out of 22 working days. He is not eligible for full salary.
So, I stand firm with my earlier statement.