Navratri is nine nights given to the Devi in her nine forms. The Durga Saptshati, its seven hundred verses recited in full, is the anushthan of these nights, with the Navchandi hawan offered where the seva calls for it.
The Special Pujas below run on a tithi you choose, so Ashtami and Navami outside Navratri are served the same way. They are carried at the Shaktipeeths — Gadhkalika and Harsiddhi at Ujjain, Durgakund at Kashi, and the Devi kshetras named for each form.
25 sevas at 23 Teerth Kshetras — you choose the tithi.
Chaitra and Shardiya Navratri are the two public nine-night festivals. The two Gupt Navratris, in Magha and Ashadha, are kept more quietly and are when the Das Mahavidya are worshipped — which is why the Baglamukhi and Mahavidya sevas are most often taken then.
The 25 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.