Few things in this life are certain...death, taxes, and 420 seed sales. Whether the sale will meet your fantasy remains to be seen.
I wouldn't expect a 30-40% across the board sale. Tiered sales would be more likely, where some breeder seeds are at each discount tier. For across the board discounts, you're much more likely to see 20-25%. Your local seed bank owner still needs some meat on the bone so he/she can eat too.
Your best bet is to get on mailing lists at seed banks you frequent so you can find out about any & all promos.