When she was 11m. It was because when she was almost a year, H and I had a night away planned for our anniversary (arranged since she was 6m). At this point Izzy would only feed in the night if she was teething or ill, but she was waking a lot in the night and we would usually co-sleep instead of trying to settle her. She was absolutely fine, and my mum ended up just getting into the guest bed with her, pulling the travel cot to the side as a makeshift bed guard.
You said December, yes (sorry, can't see right now)? That is lots of time, he could possibly stop feeding at night by the end of this month. And if not, well, you can keep EBM in the fridge for 3 months, so you could start pumping in Sept and have plenty to leave with your mum by Dec.
Can I suggest that you do a trial run a couple of weeks before hand? This is what we did - we left her with my parents for a night 2 weeks before. I'm really glad we did this as it reassured me that she and I could both cope apart - and it made it a little easier when we had to leave her for our "proper" night away. The first time, I bf'ed her beforeI left and when my mum came up with her ff to take over I literally couldn't let her go, I just held her and sobbed. The second time, it wasn't easy by any means but a lot more so than the first time, and meant our night away wasn't ruined by me worrying as much as I might have done.
(Sorry, this has turned out really long!)