After retrieving Auriel's Bow, talk to Gelebor about the ammo for the bow, Sunhallowed Arrows. He tells you that he can make Sunhallowed Arrows for you if you bring him Elven Arrows. You start off with 12 Sunhallowed Arrows that you got with the bow. You also found 12 Elven Arrows, which you can give to Gelebor to get more Sunhallowed arrows. Additional Elven Arrows you can get as loot in various dungeons, buy in weapon shops, or obtain by killing elven enemies such as Thalmor Soldiers. A store that has been confirmed to hold Elven Arrows in its stock is Fletcher, west of Solitude Gate.

Sunhallowed Elven Arrows in Skyrim

Instead of giving the Elven Arrows to Knight-Paladin Gelebor, you could give them to Serana. She adds her vampire blood to them producing Bloodcursed Elven Arrows. If you equip the bow and arm it with the Bloodcursed Arrows, then aim it at the sun and shoot, you will extinguish the sun for one day.

Speak to Serana. She says it is time to face her father, Lord Harkon. She suggests you go speak to Isran first though, who can provide some help. Fast Travel to Fort Dawnguard. Talk to Isran and tell him you need his help:

Isran: The bow... you have Auriel's Bow. I've heard it described in tales but I could never have imagined its beauty. Indeed, the day hasn't been won while Harkon still walks Tamriel. But what of Serana? Can she be trusted to lift a blade against her own kind? Her own family?

Tell Isran that you trust Serana to do the right thing. Isran tells you he will address the Dawnguard before you set off. The men deserve to know that we've finally gained the upper hand.

Dawnguards attending the speech include Celann, Gunmar, Durak, Sorine Jurard, Dexion Evicus, and Agmaer.

Isran's Speech
Everyone! Gather round! Come on then, we haven't got all day (everyone gathers in main hall). For too long we've allowed these vampires to poison the night and kill our people! Now we finally have the means to strike back! We now have Auriel's Bow. The gods themselves have favored us and we must answer with action! The time has come to finally put an end to Harkon and his unholy prophecy! We will march on their lair and destroy those wretched abominations so they can no longer corrupt our world.! This is our fight, and this is our fate (Applause). This is the time of the Dawnguard!

Isran tells you to meet him in front of Castle Volkihar.