Pashupatinath is worth visiting any time of year, but the most extraordinary time to come is Maha Shivaratri (usually February or March), the major festival dedicated to Shiva. Hundreds of thousands of pilgrims converge on the complex, and sadhus from across South Asia gather on the eastern bank. The crowds are immense and the atmosphere electric. Come early — by midday the queues are several hours long.
For a quieter experience, any morning in the dry season (October through March) is good. The monsoon (June–September) does not prevent visiting but the river and ghats can be muddy and overcrowded.