North Vietnam and its ally China had many advantages in the war in Vietnam. They could play without their hands behind their back, they could break with tradition and they had a complete disregard for the lives of their troops. All of these advantages would come to a head with the Tet Offensive.
Tet, the lunar new year, was one of the most wildly celebrated holiday in the North. As such, nobody expected an offensive, much less a massive one on that date.
The Tet offensive involved well coordinated attacks on a number of targets in South Vietnam. The North used the cover of the holiday to mask the troop build up. Preceding the holiday, troops would be rushing home to families, so it would be much easier to hide troop movements.
The first phase of the assault would begin on January 30 of 1968 when several cities and bases containing high levels of US troops were attacked. There would also be a second phase of the attack launched in May and yet another third phase in August.
In the end, although the North would claim a victory, the US and South forces would go on to reclaim all territory lost in the attacks. In addition, they would almost completely eliminate the attacking forces.
What the North did win however, was a media war. Americans at home, seeing what the North was capable of, found it harder to imagine a quick ending to the war. After the offensive, support for the war fell dramatically.
While not a victory in the true sense of the word, the Tet Offensive was very successful for the North. It would eventually bring the Americans to the table and speed up their eventual exit from the war.