Odd request - where to find a calendar program?

[Triskavanski]

So there is a an idea I want to try, but to get organized I need to find some sort of calendar/schedule making programs or templates.

Basically its a campaign that I'm trying to map out all the things that happen each day and month, that would be typically free from Player intervention. Like what cycle of the moon it is, dates certain monsters appear, or what Billy bob the NPC might be doing in a particular day if its something important.

[CozJa]

Well, there is the Eberron Calendar (https://www.wizards.com/dnd/ec/index.htm) that may be somewhat useful, it gives you months, days, moons and planes (that may be more than you need). If you need only one moon you can just ignore the various moons and concentrate on the phase of only one of them.

[Keral]

I used donjon calendar generator. https://donjon.bin.sh/fantasy/calendar/ I found it quite useful

[jmax]

Somewhat related - if you're in an Earth-like setting, Weather Underground (https://www.wunderground.com/history/) has historical weather information by date for much of the planet going back a few decades. You can use that to pre-determine what the weather will be like. You know, assuming the characters don't change (www.d20srd.org/srd/spells/controlWeather.htm) it.

If you pick a starting time and analogous locations in your setting to places with similar geography on Earth, you can know all weather past and future for at least a decade in each direction without your players being any the wiser.

There are probably a bunch of other sites with the same information, since it's mostly government data, but that's the one I know off the top of my head and have used in the past.