In the off season, whenever that happens to be for Switzerland. Little things such as whether or not it's school holidays can also effect the price of the ticket/s.
The cheapest time of the year to fly to El salvador is from February to March and then again in May, and then from August to October.
about February =)
spring break
when the kids are at school
during Ausgust or October
March
ALL YEAR LONG
There is no such cheapest time of the year. You just has to book well in advance for your flights and you get a best deal. Say almost one or two months in advance and try to fly on Saturday you will get chap air deals.
I think... November
If you want to fly cheap book your seats 6 months earlyer
January and February are considered out of season and are generally the lowest cost.
June through to October - excluding School Summer Holidays.