Unfortunately, it'd seem that for many of us "tell" is the operative word, as in being told about a fait accompli as opposed to having a discussion.
I tend to get "I've invited SD down next weekend, is that okay?" .... and feel like saying "no it's not you should have discussed it first" but that'd go down like a lead balloon, as there's only one acceptable answer to that question isn't there, which makes me think why bother "asking" me at all. It's clever really, because if I complain I'm not consulted, he can then claim he does ask me - but usually only as described above, and after SD has already been invited !
I think Bluehen is right - many men get the hump at asking "permission" to see their own children, but the fact remains that non resident kids do change the dynamics in a household - the same as any other person would if they don't normally live with you .... and there is a whole heap of stuff to be taken into consideration like extra shopping (and adjusting meal plans because stepkids don't like what you'd otherwise planned to eat), cleaning rooms, making up beds, making arrangements (or not) with friends and family which may be affected by their arrival, the cost of collecting and returning them etc etc. There is also no getting away from the fact that however much we support our partners, not all of us, on a personal level are filled with joy at the prospect of contact for a wide variety of reasons - such as bad behaviour, lack of discipline, lack of privacy, disappointment at having to ditch or postpone non child friendly plans ... and therefore, having contact thrown at you last moment without any discussion can be especially grating. Whereas a bit of notice when you know what's going to be happening on a particular date allows you to come to terms with it in advance, make plans accordingly, budget accordingly and so on.
It is all about courtesy. There would be very few occasions I'd veer veto a stepkid visit - the only time I would do so would be if DP had forgotten about existing plans we'd made which they couldn't be included in - so why the heck he can't always discuss it with me beforehand I don't know. We have a strict food budget which I work hard at to ensure we mostly have healthy appetising food - but I find it very stressful to discover at very short notice I have extra mouths to feed for example, which might not be so much of a problem if you could just make do with extra rice, bread or potato but when you have stepkids who are very fussy it's rarely that easy as they wouldn't even eat the main part of the meal.
I have an adult child who no longer lives with us and I certainly check with DP before they come round - and that'd be for a few hours usually and just one meal - because it's the polite thing to do. And that's even though he (DP) wouldn't be personally put out because he doesn't do any cooking!
I do also think, at least if my DP and his attitude is anything to go by, that some non res dads feel very "hard done by" because they don't live with all their children, and consequently justify not consulting their partners because they "only" see them for a relatively small percentage of time and therefore they're "entitled" to that no questions asked and no consideration for the ensuing impact on anyone else. And while everyone's situation is individual I also know that when DP and his ex decided to split, he made no effort to have the kids live with him and in fact rather enjoyed the fact he could "please himself" (sic) most of the month and then "have fun" with the kids EOW. I know this because this is what he told me when we first met! .... so, I do think it's a bit rich now to adopt this hard done by attitude when he could have pushed e.g. for 50:50 care way back when.
Thankfully this is less of a regular issue for me now as stepkids are much older but when "contact" (is it right to still describe socialising with older kids as "contact" anyway - I don't call it "contact" when I see my mum ?!) does happen, DP still has a nasty habit of me being the last to know. Often he'll have told MIL about an impending "contact" visit before I get to hear of it which makes me extra mad because potentially that also means meeting up with her or having her to stay as well.
Having said all that however, I do think that even my thick skinned, rather arrogant (IMO) DP would know where to draw the line and I don't think he'd dare dump a month's contact on me with (lots of) conversation first (emergencies excepted). There's no two ways about it that's going to have a significant impact on the whole house and no-one in their right mind would think it okay for you not to have been consulted OP. Will he be taking a whole month off work ? .... if not, sorry, but as you weren't asked, you won't be babysitting and he'll need to make alternative arrangements. Will he be doing all the extra washing and ironing required ? Will he be clearing up after them, cleaning the bath and the loo ? Does he have extra money to pay for food and entertainment without any impact on regular essential expenses ? (TBH, unless you're rolling in it, I'd feel very unhappy at continuing to give his ex the usual sum of CM for that month. Yes, she'll still have some regular overheads obviously but won't have the expense of food, entertainment and transport for the kids while they're away) ..... hopefully the answers to all of those questions is yes or else I'd be going ballistic .... and if it were at all possible, I'd be voting with my feet and taking a month's "contact" with my own parents by going to stay with them (appreciate not everyone can do this).
Whereas if he'd approached you some time ago, to mull over the idea, to reassure you he'd pull his weight, that you wouldn't get any extra work and be prepared to compromise if you had any reasonable objections, e.g. by reducing stay slightly or breaking it up by spending time with grandparents, then you wouldn't need to be posting here .......