great customer appreciation sale
A large-scale sale event held by a business to thank customers, often at year-end, on a company anniversary, or similar occasions. The term is a compound of 大 (big), 感謝 (thanks), and 祭 (festival).
At the year-end great customer appreciation sale, home appliances were half price.
To commemorate our 50th anniversary, we will hold a great customer appreciation sale.
感謝祭 is a more general 'thanksgiving festival' or 'appreciation event', while 大感謝祭 specifically implies a large-scale customer appreciation sale.
セール is a general loanword for 'sale', while 大感謝祭 is a specific Japanese marketing term for a grand appreciation sale event.
Compound of 大 (dai, 'big'), 感謝 (kansha, 'thanks'), and 祭 (sai, 'festival'). The term is a modern commercial coinage for large-scale customer appreciation sales.