Help!! Understanding ovulation!

Ta
I'm really new to TTC and need help understanding ovulation as I've confused myself!!
So I had a negative OPK on the 17/12, didn't test on the 18/12, strong positive on the 19/12 at 6am, weaker positive this morning at 6am and negative just now 4.30pm.
I had intercourse twice yesterday but that's it since 17/12..
Have I misunderstood when to have intercourse and missed the optimum time to do it in my window or should we try tonight too? 
(I know we should've had it more but my partner was away all weekend)