如果想12月十幾號返HK
大約要幾時BOOK機票??
因為聽講好難BOOK
好多時冇機位返...

IMHO, you can start searching about 3 months before your departure date. If you are returning from New York, it should not be too difficult. However, I don't know about the situation in SFO or LA

If you want to secure the seat reservation on that particular, make it 9 months before departure  (Last times I hold a 1 yr valid ticket but airline say seat reservation in advance is 9 months)