We've talked now about the counting of years, and we've talked about the calendar for your campaign, but we need one more detail in order to bring it all together. You know how many months you need, and how many years have passed in the eras of your world, but they still need to be brought to life, they still need names.
As a minimum you'll probably want a name for the week days and months and some sort of naming convention for different eras in your setting. This will allow you to present time in a familiar format and convey some basic information in a quick sentence. There are many great ways to name these increments of time, but I'll present a few of my favourites here to give some ideas as a starting point:
Rulers: If your campaign focuses largely on an area which has a long history of unified rulers or monarchs than each month could be named after a distinct ruler. From there one could name each day of the week after an aspect of the kingdom that the monarch manages. For example: Mercantile day or Merchday, Taxday, Warsday, Thanesday, Fieldsday, Statesday, and Spiritsday. And you could break your eras into two distinct chunks such as BDM (Before the Dawn of Man.) and AMD (After Man's Dawn). Of course you could mix this formula up easily by making each monarch's reign its own group of years and then name each month for a different famous duke, prince or princess
Farmers calendar: Common folk worry about common things, and they may well track common things as a method for keeping time. One might see a kingdom or even large stretches of environmentally similar areas using a calendar that tracks common farm life. In this system the months might be broken down based on how warm or cold it is, or the types of crops grown during those months. One would see "Month of the snow yam." or "Whetmonth" or "Month of corn". And the days of the week could be broken into Important tasks. Milkday, Tillday, Weed day, Plowsday, Fenceday, Watersday, and Restday. Of course these days would not be hard and fast rules, a chicken farmer obviously has work to do on Milkday, and one doesn't need to till their field every week. The idea is the naming serves as a general reflection of the tasks in a farmer's life. Although having whole communities out doing similar work all at the same time does lead to some interesting options. The eras of such a timekeeping system aren't likely to matter much to the common folk as few have much stake in any group of years besides the ones they live in.
Magic Calendar: It's not too hard to imagine spellcasters being in charge of making a lot of the most important choices in a region, kingdom, or world. After all, they are typically the smartest, wisest, and most persuasive of people. If spellcasters decided to egotistically name the passages of time after themselves we might see this calendar used. Months could each be named after a type of spellcaster. The Month of the oracle, the Month of the Priest, the Month of the Magus and so on. Whereas a week might be broken into eight days: Abjurday, Conjurday, Divinday, Evoday, Enchanday, Illuday, Necroday, and Transday. Of course a more puritanical calendar might exclude Necroday for an even six weeks. To take this a step farther one could include in their setting that on each day the schools spells become stronger getting +1 to effective caster level or other bonuses. In a calendar like this the Eras may be tracked with the discovery of new arcane methods. "In the era of truespeech" or "In the time of the occult" or "During the reign of Psions" or such things as that.
Divine Calendar: It should be little surprise to anyone that even in worlds with a myriad of religions, faith has a huge impact on history and society. In a world in which gods are provably, demonstrably real it is likely to draw even more from its gods. A divine Calendar takes its names from the Gods and demigods of the setting. Months like "Pelortober" Or "Nerulvember" or "Nurgle Month" or "Baneuary" may fill your setting. Or maybe you can pick names that don't sound terrible. Your weekdays could then be named after famous demigods, or even perhaps specific religeous rituals. "Monastary day." followed by "Tunesday" and then "Worship day" and "Theurge day" and "Fasting day". The eras in a setting can be tied to significant religious events, such as "635WGW" (When gods walked), or 235 GF (God fell), or other events tied to your mythology or cosmology.
Numerical calendar: Wouldn't it be nice is everything just made sense? The Months of your year could be so perfect. Evenly numbered months telling you exactly which number it is. "Primus." "Deus" "Trinitary" "Quatober." "Quince" "Sextus" "September" "October." "November." "December". In this system one might not even bother with naming days of the week, they aren't strictly necessary after all. Alternatively you could have a binary of "Workday, workday workday, holiday" Every four days a new cycle. And the Eras may simply be increments of 1,000 or 10,000 years, or you may not have any at all, simply using an ever increasing number of years instead.
Realistic calendar: Just take a mouthful of the different ideas above and spit them up on paper, edit absolutely nothing and BAM, you have a realistic and likely calendar. Unfortunately people don't always agree on things, and often times kingdoms are conquered and absorbed and some ideas pass on and some aren't. One look at the Gregorian calendar will tell you that history doesn't always make sense. Name one month after a god, name a weekday after some king's favourite horse, have an era that was a 2 year long gap in which all the recorded history was just forgotten or burned, just take several good ideas and bash them together with a crowbar into a hot mess.
One more thing to take note of. My recommendation when it comes to months and weekdays, and I'm certainly not the first to suggest this, is to try to keep the names somewhat familiar. Many of the above examples may have had you intuit which days of the week are analogous with our own week, and this is very intentional. Even though knowing "Today is Tillday, which is just like Tuesday" isn't inherently crucial, being able to say "My magic item won't be finished until Fieldsday" without having to stop and check and ask the DM about the calendar, can go a huge way to showing the detail and immersion a player can enjoy by having a relatable weekly cycle.
Hopefully this all has helped you out in determining the months and weekdays and eras of the calendar in your game, and inspired you to create a novel, exciting, or at least functional, calendar of your own campaign so you always know what day tomorrow is.