The Spider Moon mod adds a new moon to Enhanced Celestials, increasing spawns of spiders.

During the rare Spider Moon event, the weight of spider spawns will be greatly increased, and during a Super Spider Moon, all mob spawns will increase by a large amount as well, leading to a world overrun by creepy crawlies.

All credit for Enhanced Celestials goes to the original author. CorgiTaco

For added creepy-crawliness, try it with Nyf's Spiders