To be fuming?

Poor woman. She must be desperate or lonely to not tell you until the last minute that she's staying another night. Have you both bothered to ask why she wants to stay somewhere she knows she's not wanted.

Well for the extra day she gets toast for dinner

Is there a reason she may want to stay longer, apart from upsetting you.
is she perhaps lonely and wanting to spend time with her son and grandkids?

My mil is a bit irritating, but she's quite fond if her son, and he likes her. My DC have only one grandparent who is capable of spoiling them (with attention AND chocolate).

And I love my DH and my DC.

That was pre school runs and dog walk responsibilities though so easier to just pick up and leave.

So????

If you go, your 'd'H will have NO CHOICE but to do those things himself, and THAT will end this rubbish once and for all!!

A week every couple of months?! Have a word!

A 3 night stay every other month is a decent amount.

The OP has explained why she & her DH can’t WFH when her MIL is there and given she knows the layout of her house better than me, I’m inclined to believe her.

The OP isn’t rude or ‘horrible’. The MIL definitely needs to work on her manners.

Batshit

Tomorrow abandon all hosting duties. I would get in there early and strip the bed. She wouldn't be staying for at least 6 months after this stunt.

The way I'd look at this: MIL staying longer has created some problems for you, so she has to be part of the solutions.

Problem 1 - MIL can't be in earshot of your confidential calls, you can't take those confidential calls somewhere public, and it's too unprofessional for you to take the calls sitting on the bed. Solution: she has to make herself scarce tomorrow. Tell her SHE needs to go and sit in a coffee shop all day. She doesn't like it? That's a shame - this is what you were trying to spare her from with the originally planned departure date...

Problem 2 - Meal planning. You don't have time to plan the meal, you don't have time to go out and get the food. MIL has no plans for tomorrow - task her with sorting it. You get my drift!

Why in god's name is she there every couple of months? What in the fuck? That's batshit ridiculous. Once a year or once every 6 months, fine. But every two months? Fucking ridiculous and you need to put a stop to this right now. Say from now on she stays once every 6 months and make sure she knows if she pulls that stunt again, she'll have to stay the extra night at a motel, and get your husband to back you up. Tell your husband she goes, or YOU will leave and stay in a motel for the night.

You're a pushover and your husband is as weak as a fucking piece of limp lettuce. Pitiful. Time for you to put a stop to this, once and for all and mean it!

You absolutely must give her toast for dinner.

Sorry no but a week is not more normal and the OP doesn't sound horrible but at the end of her tether with a MIL who oversteps. Are you her MIL by chance?

I loved my husband’s family.
He had 7 siblings & a mum & dad.
We moved a lot for husband’s job.
So his family came for visits, a week at a time, always welcome - entire families. I loved having them, but it was exhausting having to cook and clean and do laundry and then have a week to clean up before the next sibling’s family came.
It’s a lot of work to host someone and a lot of energy.

I can understand why you are pissed off OP as having people to stay is stressful - I don't like it much either.

However

To those people saying she should kick her out, ban her from ever coming to stay again, etc etc.....

Do you behave like this in real life?

Sure, it's not great to ignore somebody's boundaries and over stay your welcome, even when it is your own son - but it's not actively evil - it doesn't deserve a cruel and humiliating response such as to be evicted from the house on the spot, or banned from ever returning.....

I presume OP's husband would not countenance any of this anyway. If my mom came to stay and then arranged a day longer than we wanted her to, and my husband said she had to leave on the spot, and/or she could never come again.....

It wouldn't be my mom that would be leaving that night......

Premier Inn provide WiFi surely? Pack a bag. And a bag of treats and get away op. Leave dh to host her..

I think you have a DH problem.

I'd be the one going to the Premier Inn if she stayed beyond the agreed number of days!

I can see how her presence interrupts your work environment. Could you go to Starbucks for the days she's there?

I'm being devil's advocate here, but I suspect there is far more to this than her adding on an extra night surreptitiously.

Are your parents restricted to 3 days visiting every couple of months too?

Honestly, you sound really horrible. Your MIL wants to spend time with her son and his family. Presumably she's not asking to move in or stay with you for months on end. A week seems a far more normal visit than 3 days. Do neither you or DH have offices you can travel to for work if it's hard WfH with her there (although I don't see why her presence stops you WfH tbh).

When we lived away from family, we paid for flights to come and visit

1. so they didnt incur extra expense to visit us
2. so I knew when they were leaving

Is this an option for the future @Peedoff24 ?

YANBU, I think you would be within your rights to say she can only stay every quarter.

I hope at least DH is the one who cooks for her and washes the sheets etc?