Skandapurana 1.3.31.82

1.3.31.82

caturddaśī tathā śaṃbhoḥ priyā nāstyatra saṃśayaḥ..
niśīthasaṃyutā yā tu kṛṣṇapakṣe caturddaśī..
upoṣyā sā tithiḥ śreṣṭhā śivasāyujyakāriṇī.. 33.82 ..

and the fourteenth is dear to Śambhu; of this there is no doubt. / That fourteenth in the dark fortnight which is joined with midnight / should be fasted upon as the best lunar day, bestowing union with Śiva.