Airlines start "clearing seats" 7 or more days prior to the flight. This means that they send messages on the reservation system to ask if the passenger intends to travel as per the reservation. Depending on load factors, system-generated auto-cancellation deadlines are given, and this appears on your booking as the "Ticketing Deadline". 

If the booking is cancelled, and other agents have waitlisted their passengers, your rebooked itinerary will go to the bottom of the waitlist, or have an "unable to confirm" status. 

For planned tourist trips to Europe, please lock in your fares by making the booking at least 2 months before travel, and ticket it at least 3 weeks before.

Your air ticket is valid for 14 days to 1 year from the date of departure (not from the date of ticketing, with few exceptions). There are expiry dates and travel limitations on restricted fares. But if you know your dates, nothing is lost from ticketing as early as possible. We would be pleased to advise you on this. Fares can increase exponentially on a full flight, and "sell-ups" (moving from one economy booking class to another) can be very expensive, about US$90 per class for long haul.

Do consider an APEX (advance purchase excursion) fare if you know your dates for certain, and you have your visa/immigration sorted out already. 

If the flight is not full, it is possible to ticket and fly on the same day. But it is pushing your luck. Ticketing at the Bandaranaike International Airport itself is improving, but is nowhere near international standards.
 

Some embassies require that the passenger submit a visa application up to 21 working days prior to travel, whereas others stamp a visa on the day of the application (with all times in between). Schengen visas take about 7-18 working days from the time of successful submission. If you need to obtain your visa in Delhi or Bangkok, please start at least 2 months in advance. 

For inbound tourists, the winter tourist "season" in the country is from November - February. During this time it is very difficult to obtain a room at beach resorts and ancient cities. Other times to avoid are the cricket season in Summer (unless you are visiting for the cricket!), if there is a visiting team touring the country.

 

If you are planning to visit major cities in Australia during the peak Southern Hemisphere summer of December/January, you should make payment and ticket at least 4 months in advance. 6 months in advance should guarantee you attractive fares and a good connection.

 

For outbound tourists, large festivals such as the Mardi Gras in New Orleans and Sydney, Edinburgh Festival in Scotland, the Carnaval in Rio, Oktoberfest in Munich, Dragon Boat Festival in Hong Kong and various Summer Festivals throughout Europe, cause hotels to fill up and all costs to skyrocket. Unless you are visiting for the festival / event itself (in which case a planning timeframe of 4-6 months is recommended), it is advisable to avoid these times for sightseeing of these areas. Another factor could be the large crowds and the escalation of tourist-related crime.