The website suggests those who want to visit Hanoi to book hotels within 3 months for 16% savings
Best time to book hotels for summer in Bangkok is within 3 months for 16% savings
In Barcelona- 2-7 months out for 27% savings  
In Beijing — 2-6 months out for 16% savings
In Berlin — 2-5 months out for 33% savings
In Buenos Aires — 1-4 months out for 19% savings
In Cancun — 2-4 months for 16% savings
In Cape Town — Within 3 months for 13% savings
In Dubai — Within 2 months for 40% savings
In Dublin — 2-5 months out for 14% savings
In Istanbul — Within 5 months for 29% savings
In Jakarta — Within 3 months for 39% savings
In London — 3-5 months out for 18% savings
In Marrakech — Within 4 months for 28% savings
In Moscow — 4-7 months out for 55% savings
In Mumbai — Within 3 months for 17% savings
In New York City — 2-4 months out for 25% savings
In Orlando — 1-4 months for 10% savings
In Paris — Within 4 months for 32% savings
In Prague — 2-5 months for 33% savings