The Gloranthan calendar is different from our own, but not so much so that it is unrecognizable. Each year is made up of five seasons, each of which consists of two months that are each four weeks long. One game turn takes up one month, so you have two actions each season. After the end of Storm Season, there is a two-week long Sacred Time, during which you will receive omens and can allocate clan magic for the coming year.
The first season in the year, Sea Season is the Gloranthan analogue to spring. During Sea Season, your farmers are engaged in sowing crops. It is possible to raid during Sea Season, but doing so is costly, as taking your farmers away from their fields will severely restrict your food supply. Other clans won't raid you during Sea Season either, so it's safe to send out missions with large escorts.
Fire Season is the Gloranthan analogue to summer. As the weather is clear and the farmers are not needed in the field, Fire Season is the traditional time for raids, and even if you do not launch raids in Fire Season yourself, you should be alert for other clans raiding you. Unless you are raiding, it is best to keep all your Swords on hand for the duration of the season. And if you do raid, make sure that some of your Bows stay home to guard your herds!
Earth Season is the Gloranthan analogue to autumn. In Earth Season, your farmers harvest the crops you planted in Sea Season, giving you an annual supply of food. As with Sea Season, all hands are needed in the field during Earth Season, so raiding is costly, and you can expect that other clans will not raid you.
Dark Season is the Gloranthan analogue to deep winter. In Dark Season, your farmers have no particular obligations, but the weather is harsh and movement is difficult. It is almost impossible to send out missions or raids during Dark Season - they will almost certainly be turned back by the weather (unless you bargain for the aid of the Pine or Winter Fox spirits, respectively). Offering sacrifices, conducting rituals and ventures, constructing fortifications and temples, and exploring your own territory, however, are still possible.
Other clans are just as restricted by the weather as you are, and cannot raid you during Dark Season. Trolls, however, can raid you in Dark Season just fine.
Storm Season is the Gloranthan analogue to late winter. As with Fire Season, your farmers have no particular obligations during Storm Season, and the weather is clear enough to allow both raids and peaceful missions. The weather is not as reliable as Fire Season, however, and while conducting missions and raids is possible, there is still a chance that they will fail due to weather. Other clans may raid you during Storm Season, but are less likely to do so than during Fire Season