Capitalize the first word after a colon only when it begins a complete sentence.
...This is what I want to know: What day did he arrive?
...This is what I want to know: the day he arrived.
...He provided the following data: The date was the 4th at a little after 5:00.
...He provided the following data: the 4th of May at a little after 5:00.
Capitalize the first word after a dash only when the word always has to be capped. (Do not cap for a complete sentence.)
...I was going to see him -- it was Friday, June 3 -- to ask that question.
...I was going to see him -- I think it was Friday, June 3 -- to ask that question.
...Were you intending to -- was it your intention to complete this?
...Were you intending to -- I want to know about your intention to complete this.
Happy punctuating!
Margie