In classic D&D, druids must gather mistletoe to enact their magic. That idea seems to be based on a few stray sentences written by Pliny the Elder, 2000 years ago, which puts it way ahead of just about every other "ancient" druid tradition that modern-day folks have tried to re-enact.
Although some D&D players may have chafed at the strange rules and rituals for gathering mistletoe, in most versions it's actually less restrictive than what plain old wizards had to go through... Not only did the garden variety magic-user have to lug around a heavy spell book (what adventurer wants nightly homework?), but each spell had its own required "material components" (e.g., pinch of talcum powder to detect invisibility; a chunk of amber to cast a lightning bolt). Compared with all that, having to gather mistletoe at Midsummer Eve with a gold or silver sickle doesn't seem so bad.
In Homebrew '82, I decided to remove the cumbersome books and material components for magic-users and replace them with the requirement to own and use a wand. Fans of Harry Potter know the possibilities of Priori Incantatem, the mysteries of Ollivander's shop, and what can go wrong if you don't have some Spell-o-tape. I've already talked a bit more about this proposed magic system here. If you don't have a proper wand, it's a flat -5 penalty to the d20 "to-hit" type die roll to see if the spell succeeds.
I suppose that a similar thing could be done for druids: if you're lacking in proper mistletoe, you get a similar penalty. However, I also think that the lore about gathering the stuff at the proper time should play into these penalties and bonuses. Thus, have a look at a trial version of a Lunar Phase Mistletoe Gathering table...
|Click to polymorph|
Note that I've listed four types of mistletoe, each with a different monthly phase. In reality, the Celtic druids only had access to the classical European variety -- viscum album with the white berries -- but in a fantasy game world, the more the merrier.
European Mistletoe peaks in magical freshness on the sixth day of the month (as Pliny described) counted by starting at the new moon. If you're just a day late, though, it goes bad quite quickly.
Dwarf Mistletoe is best when gathered under the bright full moon (it's hard to find in the dark!) and isn't so great when cut under the new moon. The related Red Mistletoe has a similar bonus under the full moon, is worse than its dwarfy cousin in the complete darkness, and has some unexpected oomph when the sharp, sickle-shaped crescent is in the sky.
Ghost Mistletoe is the rarest variety (maybe because I totally made it up), but it's the most reliable in terms of bonuses over the course of the month. Avoid the new moon, though!