From the 1976 DC Calendar.

January
11: Birthday of Andrew Nolan (Ferro Lad)
12: Birthday of Brin Londo (Timber Wolf)
28: Birthday of Gim Allon (Colossal Boy)

February
5: Birthday of Tasmia Mallor (Shadow Lass)
8: Birthday of Tenzil Kem (Matter-Eater Lad)
11: Birthday of Rokk Krinn (Cosmic Boy)
21: Birthday of Jo Nah (Ultra Boy)
24: Birthday of Dirk Morgna (Sun Boy)
29: Birthday of Kal-El (Superboy)

March
25: Birthday of Lydda Jath (Night Girl) of the Legion of Substitute Heroes
27: Birthday of Lar Gand (Mon-El)

April
5: Birthday of Chuck Taine (Bouncing Boy)
6: Brainiac created by computers of planet Colu.
11: "Birthday" of Linda Danvers (Supergirl). Anniversary of her arrival on Earth [1976 Super DC Calendar; placed on May 18 in Action Comics #305 (1963)]
12: Birthday of Dawnstar
20: Superboy's first meeting with Mon-El
21: Birthday of Tinya Wazzo (Phantom Girl)

May
5: Birthday of Condo Arlik (Chemical King)
14: Superboy becomes Superman
16: Lana Lang given bio-ring by alien, she becomes Insect Queen
18: "Birthday" of Linda Danvers (Supergirl). Anniversary of her arrival on Earth (at 12:17 PM EST) [Action Comics #305 (1963) & Supergirl #20 (1984); placed on April 11 in 1976 Super DC Calendar]

June
3: Birthday of Salu Digby (Shrinking Violet)
18: "Birthday" of Clark Kent. Anniversary of the day he landed on Earth [Pre-Crisis version; Post-Crisis Clark Kent's "birthday" is in November]
29: Krypto lands on Earth

July
10: Birthday of Drake Burroughs (Wildfire)
13: Supergirl meets Comet, the Super-Horse
17: Birthday of Lana Lang, Superboy's girlfriend
21: Birthday of Staq Mavlen (Fire Lad) of Legion of Substitute Heroes

August
14: Supergirl adopts Streaky, the Super-Cat
15: Birthday of Ral Benem (Chorophyll Kid) of the Legion of Substitute Heroes
17: Birthday of Lois Lane
20: Birthday of Jan Arrah (Element Lad)
30: Superman builds his Fortress of Solitude

September
3: Superboy’s last day in Smallville
10: Birthday of Reep Daggle (Chameleon Boy)
18: Birthday of Val Armorr (Karate Kid)
22: Birthday of Kara Zor-El (Supergirl)

October
5: Birthday of Luornu Durgo (Duo Damsel)
22: Birthday of Thom Kallor (Star Boy)

November
10: Birthday of Garth and Ayla Ranzz (Lightning Lad & Lightning Lass / Light Lass)
19: Birthday of Lyle Norg (original Invisible Kid)
23: Birthday of Imra Ardeen (Saturn Girl)
29: Birthday of James Bartholomew (Jimmy) Olsen
30: Birthday of Dag Wentim (Stone Boy) of the Legion of Substitute Heroes

December
8: Birthday of Nura Nal (Dream Girl)
14: Birthday of Ulu Vakk (Color Kid) of the Legion of Substitute Heroes
19: Birthday of Princess Projectra
22: Birthday of Brek Bannin (Polar Boy), leader of the Legion of Substitute Heroes
30: Birthday of Querl Dox (Brainiac 5)