when should i test!

i ovulated on the 6th, last period was nov. 18th, im due dec 21st, but on the 13th i had a string of blood with some discharge and nothing else after that, i never spot (hoping implantation bleeding).. when should i take a test that will most likely be positive if I concieved? UPDATE: after dealing with the worst evap ever, af came and was super weird and getting my hopes up and now she's here to stay 😔