Richard Ratcliffe spent his second festive period apart from his wife and three-year-old daughter Gabriella - but said the situation now was "a lot more positive" than last year.
Mrs Zaghari-Ratcliffe, who is British-Iranian, was arrested in Tehran in 2016 and is serving a five-year sentence over allegations - which she denies - of planning to topple Iran's government.
Hopes for progress have been raised after a visit to the country earlier in December by Foreign Secretary Boris Johnson - who had previously come under political pressure after a gaffe about the case.
Last week it emerged she had been marked as "eligible for early release" on the Iranian judiciary's database.
Mr Ratcliffe said he had hoped his wife would be home in time for Christmas Day or Boxing Day but that this "big happy news didn't happen" and he was now in "waiting limbo".
He added, when asked if he remained optimistic about her release: "I think it is a question of when, not if.
"We put a lot of time into having her home for the Christmas holiday period (up until January 6), and if that doesn't happen my worry is there must be something blocking and more complicated than we realised.
"With each day that goes past it is another day that she has not been released, and so I need to be realistic about that.
"But I haven't given up hope that this holiday period could still deliver a late Christmas."
Mrs Zaghari-Ratcliffe was arrested after visiting her parents in Iran with her daughter and has now spent more than 600 days behind bars.
The parents are now looking after Gabriella in Tehran and Mr Ratcliffe, from Hampstead, north London, has not seen the child since then except via Skype.
Mr Ratcliffe said his wife had been "fairly upbeat" until 23 December, when it turned out her release was not about to take place, and was "very low" on Christmas Eve.
She marked her 39th birthday in prison on 26 December by cooking for her fellow prisoners.
Mr Ratcliffe said: "Fundamentally, I think the bigger picture is a lot more positive than it was this time last year.
"We are close, but we could stay being close for quite a long time."