“Have a blessed day” is definitely a put down. Whether the holier than thou, poorly educated, christo-fascist recognizes this or not. It assumes a great deal and hijacks the societal secularism of “Have a pleasant, nice, great, wonderful, etc day” by appropriating your day into their kook religion.
This point is easily proven by them being placed on the receiving end of a Satanist cheerfully waving them goodbye while saying “Have a sacrificial day!”. How could they possibly take offense? (Especially when their manual is full of witches, ghosts, demons, and plenty of sacrifices ordered by their god).

I feel the southern obliviousness to their offensive rhetoric on a daily basis here in Oklahoma as well. What I tend to do is respond with is Salaam Alakum. It basically means and peace to you. Which mirrors their intended sentiment. And is the good vibes I try to embrace as much as I can. Yet also pokes at the fact that THEY do not appreciate even good will from other religions. So why must they persist in shoving their delusions onto me?

I can tell the difference between a Christian just trying to be nice vs one being petty. I would like a snappy comeback for when someone tells me “I’ll pray for you” when they do not agree with an opinion I have. They will end an argument with me by saying “we’ll have a blessed day” or “I’ll pray for you” or something that indicates I need saving. That in my opinion deserves a snarky rebuttal.

you: "Blessed by whom?"
her: "By God of course!" (Assuming the Christian god.)
you: "Ah... vanity. That breaks a commandment."

The implied statement actually is along the lines of "may God bless you." The thing is that there's no reason to say this to the person one is wishing God's blessings upon, as it is essentially a prayer. It is done purely for vanity's sake, i.e. taking the Lord's name in vain.

I need condoms

It always shocks the crap out of me. lol I was in my 50s before I had a store clerk in New England say 'have a blessed day' - I just stared like 'what now?'... I was stumped.

I do like the return blessings someone listed below though... I mean why not? And 'The Force be with You' just can't ever be offensive right?

My response to all such similar well-wishers is, "Thanks, I'm workin' on it!"
But I also like, "May the Force be with you".

I’m in Mississippi so I get it all the time sometimes I say nothing like in long lines .. but if I have time while I’m gathering my purse or whatever,..
I normally tell them, “try have a nice day cause we’re not all Christians.”
sometimes I say “Santa clause to you too.”
But if it’s a cashier I’ve already politely told I’m not Christian and they say , but god loves you anyway.
I normally respond with “all hail Satan”
Not that I believe in a xtian form of Satan at all, but Satan as a symbol for rebellious people.. and for shock value..

Each case is different.. if it’s an old person I just smile ..
feel it out and have fun..
Other things I say in return are ;
“the force be with you.”
“ have a magical day”
“By Odins beard you too.”
“Seven blessings “(GOT)
“Doesn’t compute”
“ pixi dust”
“Tardis appreciation day”

Anything odd back..

[etymonline.com] Thank you but I've already sprinkled blood on my pagan altar

May the blessings of the universe follow you for ever!

I just take it as it is (in my opinion) meant -- to be nice. Their beliefs, being kind. No more.

Does it upset you?

In London we say Ta! its extremely short for thank-you and allows you more time to escape without mismaking yourself

Live in Joy!!

Namaste.

They mean well. I have no problem with it. I don't say God Bless you when someone sneezes. I say bless you. There is a difference.

Thanks. I'll take all the good luck you have to offer.
I guess that's better than being cursed.
Happy Halloween. (not on Halloween -- on Halloween, say Merry Happy New Year)
I hope you die the death you fear the most. (this is reserved for very special cases)

"You have a blissed day, too."

Thank you and as an atheist I always have a blessed day, I bless it every morning , you should try it.

"Thanks. I need all the blessings I can get."

I respond in the same manner as when told to "have a nice day."

"OK, I will. (pause) Or, I won't."

Thank you. You too. It was a gesture of good will, after all.

I say 'I's better than a cursed day'.

I just smile and nod my head. They mean well.

Well, there's:

"You're not the boss of me..."
"Why yes, blessed are the gays..."
"Who told you about my baby oil?"
"I will if you will, and we can compare notes..."

But in all seriousness, why? Is this the hill you want to die on?

People say that to me all the time. I usually just say "you too!" as I walk away. I also have no problem saying Merry Christmas or blessing someone when they sneeze.