Hrim, Bham, Sam, Sham, Pham, Sam, and Ksham are the Melanamantras.
The Lotus-born Brahma said,
"O Shankara, which of the new moon, the first day of the lunar fortnight, and full moon is spoken of as the Mantra's sign?"
It can be kept solid on the first day of the lunar fortnight, as well as on new moon and full moon days. There is no other choice or time limit.