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These two events appeared to be on every server at the same time. One is a "Buy 1 Get 1 Free" special menu with a variety of pricey items and some event-only items such as Anima. Whatever you bought (already at a small discount), you got doubled.
For example, you could buy 10 units of Anima, normally selling at 100 diamonds for 10 units in the Mall. The BOGO event sold this at 80 diamonds for 10 Anima. If you bought it for 80 diamonds, you received 10 + 10 free, or a total of 20 Anima or 60% off the regular price. This works out to 4 Diamonds per Anima, which is much better than buying additional Anima's Path attempts, although those attempts would also give you 50 XP, gold, and one unit of material required for an Anima upgrade.
Items in the Buy 1 Get 1 Free event also included some items for sale with Topaz only.
Possibly cleverly in tandem with Buy 1 Get 1 was an Event called "Spender Bonus", where you got additional prizes for spending 500 to 35,000 diamonds over the event period.
Since this event overlapped with the Wheel of Fortune, Lotto Tickets for the Wheel of Fortune were part of the Spender Bonus at 7000 diamonds, where you could get 5 tickets in additional to other prizes (200 Anima and 10 Mythic Hero Shards of choice). In total, just these three prizes are worth 3500 diamonds.
All in all, Buy 1 Get 1 in tandem with Spender Bonus is a very excellent event, but one that is liable to clean you out of diamonds entirely.
Remember to save enough for Domination betting (1440 diamonds) as well as various other daily expenditures such as buying Stamina and Guild Donations.
The best use of this event is to WAIT till the last day. At that point, decide how much you want to spend and try to target one of the Spender Bonus reward milestones. Then look in the Buy 1 Get 1 Free store AND elsewhere -- such as the daily Special Offer, which can give you as deals just as good -- and purchase your way up to the diamond spend milestone, AFTER regular daily expenditures.