What occurs twice in a week, once in a year but never in a day?

Irene killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

John speaks the truth only once a day in a week. Below are a few hints for you:

First: Days are Sunday, Monday and so on.
Second: One day he says, "I lie on Monday and Tuesday".
Third: On the next day, he says, "Today is either Thursday, Saturday or Sunday".
Fourth: On the next day, he says, "I lie on Wednesday and Friday".

Can you identify the day on which he speaks the truth?

A girl was sitting in her hotel room when she heard a knock on the door. She opened the door and found that a man was standing outside. The man said, "Oh! I am really sorry, I thought this was my room." He then walked through the corridor to the elevator. The girl did not know the man. She closed her door and called security asking them to apprehend the man. What made her suspicious of that man? He might have been genuinely mistaken.

How many times can you subtract the number 5 from 25?

Find the next number in the sequence

1, 2, 6, 21, 88, __?

Solve the following series:
AZ, GT, MN, __, YB

Using eight eights and addition only, can you make 1000?

The below given figure comprises of a pattern through which you can determine the missing letter. Can you push your mind to find the pattern and add the missing letter?

Which statement is true out of the following?
One statement here is false.
Two statements here are false.
Three statements here are false.