This trick must be used when we need to calculate previous date from the given date.

On 10 September 1991 was Tuesday. so What was The day on 13 March 1991.

First of all we must Know:

0th odd day=Sunday

1th odd day=Monday

2th odd day=Tuesday

3th odd day=Wednesday

4th odd day=Thursday

5th odd day=Friday

6th odd day=Saturday

STEP 1 :

First of all we calculate the total number of days between the date 13 March 1991 to 10 September 1991

Number of Days in1991
Mar 18
Apr 30
May 31
Jun 30
Jul 31
Aug 31
Sep 10
Total No. of Days181

STEP 2 :

Now we calculate the odd day number and for that we have to divide the total number of days with 7(1week=7days) and reminder will be our odd day number.

181 / 7 = 6 odd days (6 is Reminder)

STEP 3 :

As we know given day is Tuesday which is 2th odd day

Let the odd Number associated with the day on 13 March 1991 is 'X'
Tuesday on 10 September 1991 = 2 odd day
so 'X + 6 = 2'
Now we have to put the value of 'X' as after dividing 'X + 6' with '7' reminder will be '2'
so 'X = 3' and day is 'Wednesday'

Watch Youtube video of Calendar for better understanding.