A 12 to 14-hour train journey from Bangkok to Chiang Mai in Thailand's far north is a rite of passage for budgetconscious backpackers.

Now here’s the real trick: take just three additional leave days on May 6, 7, and 8, and boom—you’ve got a glorious 10-day holiday from May 3-12. That gives most in the private sector: Want to turn it ...