Tu Bishvat is the 15th day of the Hebrew month of Sh'vat. The tet has the numerical value of 9 and the vav has the numerical value of 6, so together they are 15. It is a minor holiday celebrating the "New year for trees". Tu Bishvat is often misspelled as Tu B'Shvat in English. The word Sh'vat has a "shva" under the shin. The bet prefix (meaning "in") also has a shva. Hebrew does not allow for 2 consecutive shvas at the beginning of a word so the first one under the bet becomes a chirik, roughly equivalent to the English short "i" sound.
Hear "Tu Bishvat" pronounced in Modern Hebrew by an Israeli: