Days in a week and Months in a year
A day consists of 24 hours. To calculate bigger chunks of time, a new nomenclature is introduced. 7 consecutive days are called a week. Mentioned below are days of a week:

Monday

Tuesday

Wednesday

Thursday

Friday

Saturday

Sunday
The table given below will help you understand a breakup of days in a week and months in a year.
7 days 
A week 
30/31 days 
A month 
12 Months 
A year 
What is a calendar?
There are 12 months, from January to December where January is the first month and December is the last month. A set of these 12 months is collectively called a year. A calendar represents each such year.
Why do we need a Calendar
We learnt the concept of time and understood the four units to measure time – seconds, minutes, hours and days. 60 seconds make a minute, 60 minutes make an hour, and 24 hours make a day. But what happens when we need to describe more significant chunks of time? Let’s try and explain this with an example
How about your examination which is 30 days away? How many minutes is that equal to? Let’s use the table below to calculate it.
1 Day 
24 Hours 
1 Hour 
60 Minutes 
1 Minute 
60 Seconds 
So, 30 days=30x24x60= 43200 minutes
Calculating the precise amount of minutes or seconds to your next activity which is very distant from today seems inefficient and this example shows us why we need larger units of time.
Now you know 30 days can be called a month. Isn't it more convenient to say that your exam is a month away than saying it is 43200 minutes away?
Several things are scheduled as per the time of the year or by month. For example, seasonal activities related to agriculture or festivals have a longer cycle and need to be marked on a calendar. These are not measured in hours or days, but in the more extended periods, we mentioned, like weeks and months.
How does the number of days vary in different months?
One of the most interesting things about a calendar is that although there are 12 months, all months do not have the same number of days. There are months with 30 days and also 31 days.
A leap year
February is an exception, where the total number of days varies from 2829. Once every 4 years, the month of February has 29 days instead of 28. These years are called leap years. How do you know which year would be a leap year? The year must be divisible by 4. So, if you take the example of the year 2010, it is not divisible by 4 and will have 28 days in February. But the year 2012 is divisible by 4, so it will have 29 days.
These are the number of days in a month:

January – 31

February – 28/29

March – 31

April – 30

May – 31

June – 30

July – 31

August – 31

September – 30

October – 31

November – 30

December – 31
As you see above, it seems as if the 30 days and 31 days months alternate, but it is not uniformly true throughout the year.
Tip to remember months by the number of days
The knuckles on our fingers provide a very easy way to remember which month has how many days. Fold the fingers of your left hand into a fist and start counting the months from the first knuckle of the index finger. Once you reach the knuckle of the little finger, begin counting backwards again till you reach the knuckle of the index finger. So the raised knuckles stand for 31, and the troughs between two knuckles stand for 30. If you put both left and right hands together, then you can count in a line without needing to count back again. Here is how it looks.