For example, Paris, London, or Luxembourg might be cheaper than Rome, and Chicago or Toronto might be cheaper than Detroit. For example, you may want to start your journey in London and end in Brussels, which means that you don't want to have to travel back to London just to fly home. Sometimes traveling on weekdays is cheaper than on weekends. Direct vs. non-direct. Sometimes staying one day less or more could make the difference in price.
Some airlines make cutoffs at a month and then two months, etc.
Ask about these dates because different airlines and companies have different cutoffs. Some agencies make you pay for your flight immediately, while others will let you pay some time later. Check with online travel agencies or visit airlines' Web sites. Some airlines offer special discounts for booking a reservation online, though there may be length of stay and other travel restrictions. Many of these announcements are made on Wednesdays.