Gabby It takes place 6 months after Blood of Olympus when Zeus banishes Apollo to the mortal world. Also it takes place around the time Magnus Chase takes place because Annabeth is away for a family emergency.
Serena Looks like it's set after Blood of Olympus, potentially after the Jackson and Kane crossovers as well.
This answer contains spoilers… (view spoiler)[I think it will take place after because I saw a synopsis on Facebook that said that Zeus turned Apollo into a mortal until he proves himself as punishment for 'causing the Giant War' and that Apollo goes to Camp Half-blood to seek help. I'm not sure if this is right, but I really hope so!! (hide spoiler)]