when can i buy cheap air ticket from virgin airline?

airline will always offer promotion during off-peak season, such as may,sept etc.

or u can always buy discounted ticket from travel agency.

here is some online travel agent,

www.zuji.com.hk
www.priceline.com.hk

you can go to the travel agency and ask their travel agent. They have exprience in booking ticketing and also each airline flight info.

Yes... Usually airlines will put out promotional tickets around 3months before the date of flying... And I should believe that promotional tickets for around Eastern period should be out soon...
For these promotional... You can book them via travel agency or even Virgin Atlantic website at www.virginatlantic.com and sometimes you could find promotion tickets on Virgin Atlantic website as well...

淡季
多數都是3月﹐5月或者10月