I’m charting my BBT I had a huge temp spike does that mean ovulation happened the day before the temp spike OR the day OF temp spike?

Question

bbt I had a huge temp spike does that mean ovulation happened the day before the temp spike OR the day OF temp spike?" /> 
FIRST TIME TEMPING. 
I’ve read that ovulation happens the day before the temp spike but I’ve also seen that ovulation happens the day of the temp spike so I’m just curious.