WoW’s Anniversary takes place in November and runs for two weeks with varying dates each year. In 2020 it runs from November 1 to November 22, earlier than in previous years and a week longer.
Since 2011, Blizzard has celebrated WoW’s anniversary by mailing players an item which grants you an experience and reputation bonus. Using the Celebration Package grants a 16% bonus to all experience and reputation gains through the end of the event. Unlike in previous years, now the item does not need to be reused every hour or after dying; it will disappear when used and the buff will persist throughout the event.
And this week you can stack it with the 10% reputation buffs from the Darkmoon Faire for a massive 26% (or more) boost to experience and reputation. If you can grind this week, this is the best time to do it this year.
Darkmoon Faire runs one week each month beginning on the first Sunday of the month.
There are two ways to get a 10% reputation bonus from the Darkmoon Faire. Unfortunately they are mutually exclusive and do not stack with each other. So you can only use one of them at a time.
Ride the Darkmoon Carousel at the Faire to receive [Spell #46668], which grants you a 10% reputation bonus for 5 minutes. Stay on the carousel and the buff duration will increase, until it maxes out at one hour. Then hop off and head out quickly to earn your reputation. You also get this one hour buff as soon as you exit the Darkmoon Rollercoaster.
Or, buy a bunch of Darkmoon Top Hat and take them with you. Using one of these grants you a 10% reputation bonus for one hour. The hat is an uncommon reward from a Darkmoon Game Prize, or you can buy them from Gelvas Grimegate for 10 Darkmoon Prize Tickets each. But don’t buy too many of them; they only work during the Faire week and will be lost after the Faire closes.
This is currently the most powerful reputation bonus as, due to an implementation issue, both of these buffs grant more than double (over 20%) reputation bonus to mob kills. This bug may be fixed eventually, but for now it makes the Darkmoon Faire buffs nearly mandatory for any serious reputation grind.
Shadowlands still drops worldwide on November 23 at 23:00 UTC (3 pm PST, midnight CET, 10:00 am 24th AEST) so you have plenty of time and a nice bonus to level up and grind out your reputations.