I met her someday last summer.
I met her one day last summer.
I met her sometime last summer.
Someday means ‘at an unspecified time in the future’: ‘I would like to visit Japan someday.’ Do not use it to refer to the past.
For an unspecified time in the past, use one day or sometime: ‘I met her one day last summer.’ ‘I met her sometime last summer.’
The two-word expression some day can also refer to the future, although this spelling is less common than someday: ‘I hope we meet again some day.’