Track time in your campaign.
You really should, because it helps you a lot, for many reasons such as:
As seasons come and go, your players will think they're playing in a living campaign. They'll see crops growing, fields during harvest time, leaves turning brown and falling and snow covering all things.
As already said, it's very interesting to have fights during a winter snowfall, in the middle of a very heat summer or under a heavy rain.
Events like a holyday give your players a sense of a living world.
You can have religious or natural ones, such as a Moon cycle or the day of a god's ascension (maybe your own PC's god).
You can have civil ones, such as the birthday of the King or the day of an ancient and important battles.
Festivals and holy rites are useful to start new adventures or to meet new and old NPCs.
Your PCs have a lot to do between two adventures: travelling, searching ancient lore, crafting tools or magic items, researching spells and so on.
Keeping track of the time is fundamental.
You can't do that without a proper calendar.
Create your own.
You can create a very complex calendar, one with a lot of moon cycles, many holydays and ceremones different from realm to realm.
Do it, if you want it and have a lot of free time, though, as usual, I prefer to go simple and spare time.
If you're looking for a quick but useful calendar, use that of mine:
- 12 months, with 4 weeks of 7 days each.
- 4 holydays, one for every 3 months (a season), which are celebrated throughout the world.
Create any other important days only if you need.
A simple thing will improve realism and satisfaction in your players.
A good thing on the Good DM's way.