ovulation help

Ta
I'm really new to TTC and need help understanding ovulation. 
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? 
(I know we should've had it more but my partner was away all weekend)