Civic occasions
26 civic and awareness days observed in India and by the United Nations, from National Voters Day to Constitution Day, with dates and messages that take the cause seriously.
These are the days built around an idea rather than a celebration: democracy, service, rights, remembrance of a public figure. Some are Indian in origin, like National Voters Day and Constitution Day. Others are United Nations observances that India marks alongside the rest of the world, like the International Day of Peace and United Nations Day.
Where you will actually meet them
Schools and government offices carry most of these. National Voters Day on 25 January is when new voters are handed their cards; Constitution Day on 26 November has schools read the Preamble aloud; Ambedkar Jayanti is a public holiday in many states and draws large gatherings.
The armed forces days, Army Day, Indian Navy Day, Indian Air Force Day and Armed Forces Flag Day, each have their own anniversary and their own reason, and Flag Day in particular is a fundraising day for veterans and their families rather than a parade.
Writing for a cause day
The failure mode here is the greeting that trivialises the subject. "Happy International Day for Tolerance" reads as thoughtless; so does any awareness day used as a hook for a discount.
Two things work. Say something concrete about the cause, or say nothing and do something instead: donate, register, turn up. If you are writing on behalf of a company, the safest version names what your organisation is actually doing that day rather than congratulating the world at large.
For days that remember the dead rather than mark an idea, see memorial days, where the tone is different again.