How long is Tet for?

Could you please elaborate on the duration of the Tet holiday? I understand it's a significant celebration in Vietnam, but I'm curious about the exact length of time it spans. Does it vary from year to year, or is it a fixed number of days? Also, do businesses and government offices typically close during this period, or is it more of a cultural and familial observance? Your insight into this tradition would be greatly appreciated.