Sisters 21st or partners 40th?

Why didn't you send a quick message first OP to see what she was doing on her 21st birthday? I think 99% of people that had any sort of relationship with their sister would do this. The fact you didn't, and then booked your trip on that very day, makes me think you did it on purpose as some sort of big fuck you to your sister, or maybe you just like causing drama. All of this could have been so easily avoided.

So your sister either doesn't take enough interest in you/your partner to realise they also had a big birthday around the same time, or she knows and just assumes she's more important.
If it was so important you were there she could have checked first, or let you know more in advance.
She sounds like an entitled child, go on holiday and don't buy her a gift.

This

As it is booked, I wouldn’t change it. I’d have checked with her before booking and made sure events didn’t clash but it’s done now. 21st birthdays were traditionally a huge celebration in the UK. I’m late 40’s and most of my friends had big parties for their 21st, as did I.

21 was historically the coming of age in UK!

It was a bit silly of you to book the trip if you're the type of person who going to feel really guilty over it. Personally I don’t think it’s a big deal, nor do I think you should cancel or try to reschedule. It will cost a fortune. Your sisters acting like a toddler but you should just own your decision too.

It’s beyond complicated. Plus the family lunch was a surprise so he didn’t know. So I was left to tell him and that I hadn’t been invited. I no longer speak to any of his family.

No I didn't miss that, nor did I miss that her husband isn't bothered when he celebrates. I also think she should have checked in with her sister before booking to go away over her important birthday.

21yos are still pretty immature, I’m not surprised she’s pissed off at you. 21st are massive milestone birthday and it is widely accepted that the whole family and friends all celebrate together. (at least where I live anyway)
you’ve known what date her birthday was her whole life, you were a bit silly to book a weekend away on her 21st weekend. Change the trip.

Anyway, the OP is not being unreasonable.

Partner (esp with kids together) trumps sister.

I would have been happy to move the trip as you can and because partner doesn’t mind, if she’d asked in a normal way. Giving the silent treatment would put me off.

I really don’t think that’s right. Traditionally it was the 21st in the UK because that used to be age of majority. If anything it’s old fashioned to make it the 21st a bigger deal.

Nothing to do with Americans.

The sister didn’t have to have a party either! Or could have her party another weekend as that was arranged after OP’s planned trip.

Either way, it’s completely immature behaviour for an adult woman to be giving her sister the silent treatment because she can’t attend a party.

@AlcoholicAntibioticBut they didn’t have to go away! The birthdays are on different dates! The op could have accommodated both. 40 is the older adult here! Maybe the oP is a bit too
old for parties? 18 is also celebrated but more in a club.

Sister is being a brat. 21 isn’t that big a deal. 18 and 40 are.

The birthday trip for her partner's 40th was booked months ago.

The sister is trying to get OP to put her over her partner and using manipulation to get her way. Why should she be rewarded for that by OP giving up her partner's trip for her sister, who's trying to send her on a very nasty guilt trip of her own?

Time for sister to find out she can't always get her way, especially when she resorts to an abuse tactic like the silent treatment.

Have you missed that it’s also a big birthday for OP’s partner? 40 is just as much of a milestone as 21.

Her partner is closer family than her sibling.

Why on earth would she need to check that her own sister had remembered her 21st birthday??

I wouldn't book something on a family member's birthday at all, let alone over a milestone birthday. If anyone should have checked anything it should have been OP.

I would be so hurt if my sibling staffed to go away over my special birthday (and yes 21 has always been a big thing, for generations).

I think I would try and do both so your sisters 21st party and take your DP out on the other day of the weekend then do Italy on a different weekend. Only if your DP doesn’t mind though and if you are ok with your sisters somewhat brattish behaviour in not talking to you. I would prioritise your DP if you can only celebrate one of their birthdays.

21 parties have been celebrated for a very long time! At least 60 years. It’s often together with graduation and sometimes with other friends. I think the op should at least have asked about any plans her sister might be making and their parents. As the 40th isn’t on the same day, both events were possible.

As the song goes...21 today, 21 today she's got the key of the door never been 21 before 🎵 🎶!

Her sister is being childish.

My husband/partner trumps siblings. And a holiday will always trump a party.

Your sister is being unreasonable, you can do something with her when you get home.

i don’t agree, 21 has always been massive where I’m from (Midlands UK), my grandparents (Manchester), always told us 21 was the most important birthday. My daughter is 21 this year and we are treating it as big as her 18th.l - last big birthday until 30!

it stems from the Key to the Door tradition (not USA). My parents and grandparents presented me with a symbolic key on my 21st.

@EzWin2 I think you have been slightly naive to have not asked your sister if she was planning anything special on her birthday weekend, especially as your partner wasn’t that fussed. She is being quite childish but I would be pretty annoyed with my sister too.

also (sorry super boring info below for context, im just so surprised people on here don’t celebrate 21 as a milestone birthday!)

FYI: The "Key to the Door" Tradition: Rooted in medieval times, 21 was historically the age a squire was dubbed a knight. In the UK and Australia, it evolved into the tradition of gifting a celebrant an actual or symbolic key to the front door. It signified that the person was now economically independent, completely mature, and free to come and go as they pleased without parental curfews

Historical Legal Significance
Before 1970, turning 21 was the official age of majority in the UK. This meant you could not vote, enter into legal contracts, get married without parental consent, or buy a house until you reached this age. While the Family Law Reform Act of 1969 lowered the age of majority to 18, the cultural tradition of the 21st birthday being a major life milestone has stubbornly remained

University Graduation: For many, turning 21 coincides with finishing a university degree, acting as a double celebration for entering the professional workforce.

Second "Big" Party: While the 18th birthday is often heavily associated with legally drinking at pubs and clubbing with teenage friends, the 21st is frequently treated as a more formal gathering that bridges family and a broader social circle.

I would have checked if my sister was having a party before I booked to go away on her birthday.
Who is looking after the children? If your parents are looking after your children did they not say anything about her wanting a party that weekend?
We've got a few shared dates in our family and we sometimes celebrate together so we just check before we book

Tell your Sister to grow up as she is an adult after all.