April to October. This period of the year corresponds to winter at the southern hemisphere, when big storms at the infamous Roaring Forties (latitudes 40 to 50 degrees south of the Equator, at the Indian Ocean) generate strong swells. These swells travel thousands of miles, loosing power along the way, until they reach Indonesian shores where they meet coral reefs, warm seas and light winds, conditions that are just perfect for surfing.  During that period – April to October – there’s an average of 1 good swell every week reaching Indonesia’s southern shores.
During the rest of the year (from November to March) it’s summer time at the Indian Ocean so not many storms happen there, meaning rare and smaller swells. Consequently, the surf at the shores exposed to the Indian Ocean is smaller and less consistent, particularly from December to February (around 1-2 medium size swells per month).
Actually March and November are transition months and there are still a few good swells, and with a bit of luck you can get great surf without the crowds. If you are looking for “smoother” and absolutely empty waves, March and November are a great choice for a boat trip in Sumatra.