Not the weekly heroic, the weekly nightfall. Upon completing it, you get bonus xp for everything you do untill it resets on the next tuesday. For example, instead of getting 25 crucible rep for winning, i get 31 (or 32 can't remember) this also applies to XP bonuses for lvling up your gear and weapons and even glimmer. It's also a good idea to save your shards for raid gear, as it's the only itmes in the game that will allow you to reach lvl 30. The advantage of raid gear is it gives you the same light bonus and defense as exotic pieces. Where all the other legendary gear isn't as good. So, good move on saving your shards!