Skandapurana 16.24.40

16.24.40

māsaiścaturbhiryatpuṇyaṃ vidhinā''pūjya śaṃkaram ..
kārttikyāṃ sa labhetpuṇyaṃ caitryāṃ taddviguṇaṃ smṛtam ..
puṇyametattu phālgunyāmāṣāḍhyāmevameva tu .. 7.1.24.40 ..

Whatever merit is obtained by worshiping Śaṃkara according to rule for four months, / that merit he obtains on the full-moon day of Kārttika; on that of Caitra it is considered twofold; / and this same merit is likewise [obtained] on the full-moon day of Phālguna and on that of Āṣāḍha.