NO: “Have a blast day!”

YES: “Have a blast!” = an idiomatic expression used to wish someone to have a great time, i.e. on his/her birthday, at a party/event, on a vacation, etc.

The normal definition of ‘blast’ (noun) is a ‘sudden, violent gust of wind; a very loud noise’, such as in “The blast from the gas explosion shattered windows for several blocks.”

But as an idiom/slang, ‘a blast’ (always in the singular form) means ‘a lot of fun; a wonderful, thrilling time’. Other examples of correct usage:

• “I had a blast at the concert last night!”
• “That road trip was such a blast!”

Or maybe you mean to say: “Have a blessed day!” ? (and somehow ‘blessed’ is mistaken for ‘blast’).