大促销

The full form of 大促 (the e-commerce shorthand). 大促销 is the older, broader term — used for both online events and physical shop sales like a department-store year-end clearance. 大促 is overwhelmingly online; 大促销 covers both.

grammar

大 (big / major) + 促销 (promotion / sales push). 促销 itself is verb-object: 促 'urge / push' + 销 'sell'. Compare related terms: 打折 (discount), 优惠 (preferential offer), 特价 (special price).