It was a nice warm day today.
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of warm? It's warm as well! Though this is for a warm weather or temperature, not some warm food or something like that, that would be 温かい, a different あたたかい. Confusing, right?
The reading is the same like 温かい, which also means warm, though it refers to a warm thing unlike 暖かい which refers to a warm temperature or place.
*Note that あったかい is for spoken Japanese. The "correct" reading is still あたたかい. But we've added this reading too for you chattygattors out there.