Nami's Zenith starts on noon on the 20th of Nami's Zodiac month and extends through the 21st. This coincides with the 20th and 21st of the month of Verdance on the secular calendar.
"Most places have their main annual Nami festival at different times of the year. Nami likes to be unpredictable so those who wish to honor Nami usually do so at seemingly random times, far outside of Nami's Zodiac Month. Even outside of Nami's faithful, most Scarterrans view the Zodiac calendar as setting guidelines, not rules, but some people are sticklers. It is hard enough to get the sticklers to acknowledge Nami at all, so the more hidebound places prefer to honor Nami on Nami's Zenith. Which places are those? The Elven Empire, Nishi, Khemarok. basically anywhere where the Keepers are large and in charge. They don't like to let us Rovers rove around unchecked, but at least they give us leeway on these two days. What do they do in these places? The same thing everyone else does. They drink, they sing, they chase members of the opposite sex. They just do it with even more gusto because they only get these two days to enjoy themselves as opposed to be able to relax throughout the whole calendar year. Other places have a secondary party on Nami's Zenith in addition to their main Nami festival at some other point, so we Rovers are pretty busy today." -@Nora dwarven Circuit Priestess of Nami