According to the official Japanese Pokemon GO Twitter account, the current Raid Pass Box sale is scheduled to end on August 21st.
Pokémon GO 1周年を記念したショップでのセールは、8月21日（月）に終了を予定しています。お求めはお早めに！
— Pokémon GO Japan (@PokemonGOAppJP) August 18, 2017
Translation (via Bing translate):
Sale in the shop commemorates anniversary of Pokémon GO 1 is expect to finish (Monday) to 8/21. You can purchase them soon!
Although rumored, it’s not yet clear if the sale will be extended to August 31, covering the entire duration of Legendary Bird Raids. In any case, we recommend buying (if you plan to buy) Raid Passes before Monday, August 21st.
As a reminder, the current sale offers a large amount of discounted Premium Raid Passes and additional items:
|Special Box||Great Box||Ultra Box|
|Raid Passes||6 x Premium Raid Pass||12 x Premium Raid Pass||16 x Premium Raid Pass|
|Berries||10 x Pinap Berry||20 x Pinap Berry||30 x Pinap Berry|
|Modules||–||10 x Lure Module||20 x Lure Module|
|Cost||480 Coins||980 Coins||1480 Coins|