When writing in a diary is it correct to say, 'today was Sunday'? Since it's still Sunday, shouldn't it be 'Today is Sunday'?
Whatever. Nobody but you and a nosey individual will read from it.
no it is was because when you read back it wont be sunday anymore
^ agreed So just put the date and underneath it put Sunday.
From a grammar point of view it would be "it is" since you are writing in the first person present. But since its your diary you can do it any way your comfortable with.