There's just one niggling worry, as I go about the preparations.
I hope we don't have a repeat of last year.
We had two little gatecrashers to his 5th birthday bash - the sisters of his very good friend.
Despite explaining there wasn't room for siblings, his mum decided to bring them along anyway.
This might not have been so bad if it were just cake and party games at our house.
But I'd hired a hall with a bouncy castle, and paid per head for the food.
I'd also spent a fair bit on entertainment - a petting zoo, which stipulated a maximum number of children, or the animals would get scared.
So my son chose very carefully and our guest list took us to that maximum number.
Then this mum strolled in with three kids, instead of one.
"The girls aren't here officially," she assured me. "They'll just stay with me."
How a three-year-old and her seven-year-old sister were supposed to just stand there and watch all the fun was beyond me.
The party was in a hall at our local leisure centre, so I suggested the mum take her girls to the café on site, and I'd watch over her son. She shook her head.
Maybe I'm in the minority, but before becoming parents, adults didn't turn up to parties with a couple of unannounced guests. That would be rude.
So how does all of this social etiquette go out of the window, when we have kids?
Yes, there are sometimes babysitting issues. But I still reckon a few weeks' notice of a party gives enough time to make arrangements – or speak to the host before the day.
Babysitting was not an issue for my little gatecrashers, as a few enquiries to their mum revealed.
"Oh my husband's at home enjoying the peace and quiet," she grinned, as I silently seethed.
I nodded the girls towards the bouncy castle, hoping they'd stay there for the rest of the celebrations.
But once the food was served, they raced over to the table, standing behind their brother and grabbing at the sandwiches.
Then, it was time to move into the hall next door, to enjoy the petting zoo. Nothing was done to hold them back, as our little invaders filed in with the rest of the guests.
As luck would have it, two party goers had to leave early. So we didn't go over our maximum numbers.
But I was still angry with the mum, who had no idea whether her daughters were causing me any extra hassle or expense – and she didn't ask.
Parents should know better than to put party hosts in this situation. An uninvited sibling is the worst kind of Catch 22. Any way you react is going to cause upset.
Ask them to leave, and you're disappointing an innocent child (or in my case two).
You're also risking a bust-up with the parent, not to mention the possibility the sibling you've invited might also have to leave.
So your only option is to seethe quietly and take the rudeness on the chin.
In my case, there was no monetary loss. But mum-of-three Ruth had her party budget completely blown by uninvited guests.
She explains: "I'm a single parent and saved for months so my son could have five friends for pizza and go-karting.
"I was ready to pay an entry fee and buy food for six children. But three siblings showed up.
I couldn't believe it. One of the parents responsible didn't even apologise. She said nothing.
The other used babysitting as an excuse. But she could have easily dropped off her invited child and returned later.
"I didn't want a confrontation on my son's day, so I had to pay 50 per cent over my budget.
One of the mums had the cheek to look miffed when I didn't have a party bag for the two extra kids she'd brought with her."
Sam, a mum-of-one, always makes allowances for unexpected guests at her daughter's parties.
"It's happened to me twice now, and I'm resigned to it," she says. "I've even found myself watching someone's two-year-old little brother, when I should have been lighting candles and taking photographs of my little girl enjoying her day.
"There's always an excuse – and the parents know you can't argue with them. So I have a contingency fund, and I order a few extra party bags just in case."
Personally, I can't shrug off such rudeness. If it happens to us again, my son will be forgoing a party next year.
I'd rather treat him to extra presents, or a family day out, than pay for someone's kids to enjoy a celebration they weren't invited to – at my expense.
What do you think? Has this happened to you?
Is it just the way these things happen, especially when parents don't have options, or rudeness?
More on Parentdish: Why I won't be throwing a birthday party for my son