According to the official Japanese Pokemon GO Twitter account, the current Raid Pass Box sale is scheduled to end on August 21st.

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
Icon
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