There is a 5% discount for all items every month on the 16th.
There is a 5% price reduction for all items on the 16th of every month.
〇日は26日とします。
「毎月」→「Every month」
「値引き」→「Discount, Price reduction」
「All items」→「全品」
まとめて言いますと、
「There is a 5% discount for all items every month on the 16th.」か、
「There is a 5% price reduction for all items on the 16th of every month.」
と表現します。
どちらでも使えます。
「毎月◯日は5%値引きセールです。」= 5% off every month on the XXst/nd/rd.
「5%割引きセール」= 5% discount sale
「毎月」= every month
「XX日」= 1st, 2nd, 3rd, 4th, 5th, 6th, 7th.....12th, 20th, 21st, 22nd, 23rd, 24th など