Each lead's local time
Switch on Send in each lead's local time in the campaign's Settings, under Schedule. The days and hours you set are then read in each lead's own time zone, so a 9am email reaches New York at 9am and Dhaka at 9am.
mailzy works out the zone from a "Time zone" column, or from "Country" (with "State" for countries with several zones). You can also set it by hand on the lead. A lead whose zone can't be told uses the schedule's time zone.
Public holidays
A schedule can skip public holidays in the countries you choose. With local time on, each lead also skips its own country's holidays.
Plan send windows across time zones with the free send window planner.