It is entirely up to you; however, since you have asked, in my opinion there doesn't seem to be any hurry, we have been told that the event lasts the entire month. if you have played events before typically all prizes offered remain constant throughout the event, however, this is a multi-sectioned event, separated by time with the 1st event scheduled to end tomorrow...

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How to Get Prizes from the Emporium of Darkness

The Emporium of Darkness is a special limited-time shop that you can only access in this event. It contains cursed curios and terrifying trinkets perfect for the spooky season!
You can open the Emporium of Darkness at any time. To do so, approach the shop node next to the Gatekeeper. Tap on it and select “Emporium of Darkness”. Here you can see all the items on offer. If you have enough relics and want to buy an item, select it and tap “Buy”.

The prizes in the Emporium of Darkness include unique Halloween-themed clothing, accessories, and even Petkins. You can also unlock an exclusive badge based on your faction to wear proudly next to your username!

....I do not foresee, (and this is an assumption on my part, so take this as you will) the LKWD changing the available prizes from the Emporium of Darkness, adding to them maybe but not removing anything.

So at this time, I see no danger of simply holding off and saving those relics for the time being (definitely spend them before the end of the month, but no harm in holding off until the next event segment.).