This number is mostly associated with the Vocaloid Hatsune Miku, since 39 can also be read as such ('mi ku').
The number can also be read as 'san ku', leading to many Miku/Zaku crossovers.
Can also be read as 'san kyuu', which sounds like 'thank you' in English. This is played on in the song 39 (Vocaloid) by sasakure.UK and DECO*27 and 39 Music (Vocaloid) by MikitoP.
Note: not all Miku posts should be tagged 39, only posts that incorporate the number.
March 9th is also the annual date of MikuPa.