Be Quick But Don’t Hurry
~Woody Allen

How much in advance do I need to book my air ticket to get the lowest airfare?
Cheap Flight Tickets
When to Book your Flight Ticket to Get Best Value for Money
One of the perennial question every traveler have in their mind is how far in advance should one book their travel ticket to get the cheap and best air ticket price. And yet there is no simple answer to this old question. However, years of studies and research have brought to fore a few time tested methods which might enable you to get the best deals on the next air ticket that you book for the holiday you are planning next year.
If you go by reports and researches done in this area, it is safe to assume that the best deals on air ticket is most likely when it is booked from 6 to 24 weeks before the departure.
But bottom line is lower airfare is not always guaranteed and you also run the risk of finding even higher fares or worse yet that flight is fully booked. Then there is always different rules (based on studies) which are applicable for domestic flights and international flights. Whereas the 42 day advance booking rule stands true for domestic stand alone air tickets in general; for international flights, the best prices are up for grabs well  in advance as far back as 24 weeks to 82 days.
Although the trends and analysis have shown that 40-50 day advance flight bookings yield best results in terms of cheap air ticket price for domestic consumer segment, it is not guaranteed that you will get the cheapest rate. One of the solutions offered is that once you book your ticket that is all and about and never look for the ticket rates. Other ways to hope for best deals is to sign up to eAlerts or SMS alert offered by almost every travel site offering cheap air tickets.
Busy travel periods like Christmas and New Year and other local/global events occurring at the places you are traveling to could also lead to very high ticket prices if you have not booked well in advance. For example prices of flights to Goa almost double during holidays season so it is also wise to book well in advance if you have planned your holiday at that particular time.
The best practice in case of events and festivals is to book as early as possible. And the golden rule to give you the best value for your money on your air ticket is:
“Whatever you do, don’t wait until the last minute.”
P.S. Do you know that it is believed that you get the cheapest flight rate on Tuesdays at around 3 PM Eastern Time (USA Domestic Travel as per Faircompare (dot) com study)