I've frozen my arse off in May and been somewhat comfortable depending on bivvy location, time, what shelter was available, whether I was using a sleeping pad, clothing choice, fatigue.
bivvy is fine at any time of the year! pick a site high up but out of the wind (very important), lie on something insulating, pack a sleeping bag good down to 3 degrees or so, you'll be fine.
Or freezing your arse off outside in an inappropriate bivvy..