The Somvars of Shravan are the days most given to Shiva in the whole year. Devotees keep the vrat, carry Ganga jal, and have the Rudrabhishek performed — the abhishek of the linga with jal, milk, honey, curd and ghee in a prescribed order, with the Rudram recited over it.
The Special Pujas below are performed on a tithi you choose, so a Somvar outside Shravan, a Pradosh, or Maha Shivratri are all served by the same sevas. They are carried at the Jyotirlingas and Shiva kshetras — Mahakaleshwar at Ujjain, Kedarnath, Omkareshwar on the Narmada, and the Mritunjay pratistha at Kashi.
27 sevas at 21 Teerth Kshetras — you choose the tithi.
Yes. Shravan Somvar is the most auspicious occasion for it, but Rudrabhishek is prescribed on every Somvar, on Pradosh twice a month, and on Shivratri. Because these sevas run on a devotee-chosen tithi, any of those days can be booked.
The 27 sevas here are Special Pujas — you choose the tithi yourself at checkout, so they are available year-round and are not tied to a fixed festival date.