Nazanin Zaghari-Ratcliffe’s family will spend Christmas day “sitting by the phone hoping” for her to be released.
The British-Iranian mother has been held in Iran for 20 months but her husband has said she could be freed within a fortnight after an Iranian database listed her as eligible for release.
Mrs Zaghari-Ratcliffe, who turns 39 on Boxing Day, is being held in Iran on spying charges - which she denies.
Her husband Richard Ratcliffe told the BBC that the "best case" was a 25 December release.
"We are sitting by the phone hoping", he said, adding, “"It's her birthday on Boxing Day so we were sort of hoping she'd be out for then".
Mrs Zaghari-Ratcliffe is still in prison today, meaning the earliest she could be released is tomorrow.
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Boris Johnson hints at progress over jailed Nazanin Zaghari-Ratcliffe after visiting Iran
Nazanin Zaghari-Ratcliffe ‘eligible for early release’ from Iran jail
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On 3 April 2016, she was arrested at an Iranian airport while travelling home with her young daughter. She has been accused of plotting against the Iranian government.
She says she took her daughter Gabriella, three, on holiday to Iran to celebrate the country's new year and visit family.
Mr Ratcliffe said he will be lighting the candles on a birthday cake for his wife whether or not she is released before her birthday. He will telephone his daughter who is being looked after by her grandparents in Iran.
"She quite likes to watch a birthday cake having its candles blown out," he said.
"[We] sort of try and defiantly celebrate if we can, and keep spirits up."
Mr Ratcliffe believes his wife's prospects had improved since her case was taken up by Foreign Secretary Boris Johnson. Mr Johnson has become more involved after he apologised for a gaffe which appeared to contradict claims she was on holiday.