"When these days were over, the King gave a banquet, lasting seven days, in the enclosed garden of the king's palace, for all the people from the least to the greatest, who were in the citadel of Susa." NIV translation
After the big party for the rich folks, the King threw another shorter party for everyone in the city. He opened his garden to show everyone how great he was.
"The garden had hangings of white and blue linen, fastened with cords of white linen and purple material to silver rings on marble pillars. There were couches of gold and silver on a mosaic pavement of porphyry, marble, mother-of-pearl and other costly stones. Wine was served in goblets of gold, each one different from the other, and the royal wine was abundant, in keeping with the king's liberality. By the king's command each guest was allowed to drink in his own way, for the king instructed all the wine stewards to serve each man what he wished." NIV translation
This wasn't your ordinary garden or your average party. The king allowed the common folks to come in and see his greatness and wealth as sort of a pep talk for the coming war. He gave them as much to drink as they wanted.
"Queen Vashti also gave a banquet for the women in the royal palace of King Xerxes." NIV translation
The women were even involved in the party although their party was separate from the men.
"On the seventh day, when King Xerxes was in high spirits from wine, he commanded the seven eunuchs who served him - Mehuman, Biztha, Harbona, Bigtha, Abagtha, Zethar and Carcas - to bring before him Queen Vashti, wearing her royal crown, in order to display her beauty to the people and nobles, for she was lovely to look at. But when the attendants delivered the king's command, Queen Vashti refused to come. Then the king became furious and burned with anger." NIV translation
So, at the second party, the king got drunk and decided to show off his wife. When the king's servants told her what the king wanted, she refused to obey and the king became mad.