The last few months have been quite hectic. You fled from the White Tower ages ago, it seems... but it's been only half a year. You've begun to get settled in Salidar. Just three days ago, Sister Egwene got raised as the Amyrlin Seat, and Sister Sheriam was installed as her Keeper. On the dawn of Danu 4, you are summoned to her...

In game terms...

This means that the player characters start their first session at the dawn of Danu 4, 999NE. Siuan Sanche has been deposed about 6 months earlier, which led to the Breaking of The Tower and the dispersal of many Aes Sedai.
The Blue Ajah gathered at Salidar and established a new sort of Tower there. The player characters are part of that group, of the 'rebel Aes Sedai'.

The Three Oaths

Before being raised to the level of Aes Sedai, each Accepted is required to swear three oaths while holding the Oath Rod, a ter'angreal that makes oaths binding. They are:

1. To speak no word that is not true.
2. To make no weapon with which one man may kill another.
3. Never to use the One Power as a weapon except against Shadowspawn, or in the last extreme defense of her own life or that of her Warder or another Aes Sedai.

These oaths were not always required, but various events before and since the Breaking caused them to be necessary. The second oath was the first adopted after the War of the Shadow. The first oath, while held to the letter, is often circumvented by careful speaking. It is believed that the last two are inviolable.

The Seven Ajah

The Aes Sedai divide themselves in seven Ajahs. These Ajahs each have political and philosophical ideas that set them apart. A brief summary (from Wikipedia) follows.

Blue Ajah

Members of the Blue Ajah focus their energy on championing worthy causes and promoting justice. They are skilled in political maneuvering and are good at administration.

Red Ajah

Members of the Red Ajah are best known for their disliking of men. "...Red, who despises [men] as though every one of them were responsible for the Breaking." (III, p271) They are self-reliant and do not depend on a Warder for protection. In fact, the Ajah steadfastly refuses to bond any Warders at all. Their mission is to hunt down and Gentle any and all men found with the ability to channel.

Green Ajah

Members of the Green Ajah were also referred to as the Battle Ajah. During the Trolloc Wars and the Breaking of the World, the Green Ajah fought amongst the ranks of the armies at all times. Their main purpose now is to wait for the Trollocs to return south once more with the coming of the Last Battle as spoken in the Prophecies of the Dragon. They are Tar Valon's equivalent and counter to Dreadlords.
The Green Ajah often have more than one Warder for protection, and are said to have a great "love for men".

Brown Ajah

Members of the Brown Ajah spend much of their time and energy in the pursuit of knowledge. They are very learned in myth and lore, and in the Prophecies of the Dragon.

Yellow Ajah

Members of the Yellow Ajah are dedicated to both the study and practice of healing with the One Power. Members of this Ajah are the most talented healers among the Aes Sedai.

White Ajah

Members of the White Ajah focus on deciphering the truth of the matter with implacable logic. They are also said to have no time in their lives for any passion whatsoever - this includes having a Warder by their side, though the Ajah does not strictly forbid the bonding as the Red does.

Grey Ajah

Members of the Grey Ajah seek harmony and agreement. As a result, they make very skilled mediators.

The calendar

Again from "The World Of Robert Jordan's The Wheel Of Time":

The current calendar is the Ferede Calendar (...) Recording years of the New Era (NE), the Farede Calendar sets the first year after the War of the Hundred Years as 1 NE. (...)
The Farede Calendar sets 10 days to the week, 28 days to the month, and 13 months to the year. The months are: Taisham, Jumara, Saban, Aine, Adar, Saaven, Amadaine, Tammaz, Maigdhal, Choren, Shaldine, Nesan, and Danu.