Skandapurana 16.14.31

16.14.31

tataḥ siddheśvaraṃ liṃgaṃ kalau khyātaṃ varānane ..
yadā somena saṃyuktā kṛṣṇā śivacaturdaśī ..
tadaiva tasya devasya darśanaṃ devi durllabham .. 31 ..

From then on, the liṅga became renowned in the Kali [age] as Siddheśvara, O fair-faced one. / When the dark fourteenth day of Śiva is joined with the Moon (Monday), / only then, O goddess, is the darśana of that god difficult to obtain.