I long ago made up a calendar for my world. On it, I have the months and days of the week, moon phases for the two moons of my world, all the major holy days that are celebrated by all large cultural groups, and a few other things.
When I start a new campaign, I go back over the calendar in the weeks leading up to the first session and add local holidays and historical days that are celebrated, then I add events that I know I want to have happen such as elections, births, deaths, assassinations, political upheavals, battles or conflicts, etc... for the first year that the campaign will run.
I then generate some weather for about the first three months or so. I use a world weather almanac to get a rough idea of what should be going on, and then I use the old charts from the Wilderness Survival Guide to add some variables.
Once all that is on the calendar, I use it to track what the PCs do each day or week of their adventuring career. I make notes on such things as "Val orders armor. Will be ready on Elanora's day, week two of Planting". Or, actually I mark "Val orders armor" on the day she does it, and on the day it will be ready, I mark "Val's armor ready".
I find these notes invaluable when the PCs want to know "when is my birthday?" (they pick a day before we begin, if they want to) and "Did we ever celebrate Midsummer?" (ummm... no, you were busy slaying dinosaurs that week...)
It is one of the things new players often comment on; apparently a lot of DMs don't even bother with "it is summer/winter" differences.