When is New Year's day?

Often people, especially those who are not familiar with the Jewish customs, get confused over the occurrence of the Jewish New Year in the seventh month of the Tishri. Though  the Bible speaks of Rosh Hashanah falling on the first day of the seventh month, the question is how people can celebrate a New Year on  the seventh month of their own calendar . 
Instead it could have celebrated on the Nissan, the first month of the Tishri. And Nissan occurs in March and April. 

Here goes an explanation:
Instead of the usual unique one, the Judaism has several "new years," each one dedicated to a specific cause. The concept, though, may appear strange at first,  has got the logic of its own. For instance, the American "new year" starts in January, but the new "school year" starts in September. Again, many businesses have "fiscal years" that start at various times of the year. So, even here we have different new years, each  book marked for a different purpose.

Similarly Jews have 4 different days to choose from:

1 Tishri:

Rosh HaShanah. This day is a celebration of the creation of the world and marks the start of a new calendar year. This will be the day we shall base our calculations on in the following sections. Sabbatical and Jubilee years begin at this time.

15 Shevat:

Tu B'shevat. The new year for trees, when fruit tithes should be brought.

1 Nisan:

New Year for Kings. Nisan is considered the first month, although it occurs 6 or 7 months after the start of the calendar year. It is used for the purpose of counting the reign of kings and months on the calendar. 

1 Elul:

New Year for Animal Tithes (Taxes).

Only the first two dates are celebrated nowadays.