sacrifice offered on anniversary
禫 centers on a ritual sacrifice offered on the anniversary of a death, a specific ceremonial observance.
禫 combines 示 (altar, ritual) with 覃, which likely contributes the sound and may relate to extending or deep meaning, but the exact historical development is uncertain.
The left side 示 suggests a ritual altar. The right side 覃 looks like a container with a lid, as if holding an offering. Picture a covered vessel placed on the altar for the anniversary sacrifice.
For タン, imagine a tan-colored cloth draped over the altar during the anniversary rite: tan -> タン.