Over 26 miles long and more people in lycra than almost the whole of the 1980s, it can only be this year's London Marathon.
With thousands of people pelting across the city and main roads closed from 7am, it's going to be a tight squeeze if you're coming to watch or cheer someone on.
But we've got a few handy tips too if you want to avoid the Tower Bridge crush this Sunday.
Track your runner
Obviously you could use something like the free Find My Friends app. But on Sunday, there'll be a link to a tracking service on the London Marathon homepage, where you can keep tabs on your runner using their race number or surname. This is a less stalker-like option if you've come to cheer on a colleague.
We don't really need to tell you this one, but we'll just remind you what Britain is like at this time of year. Keep those shoes flat, bring your waterproof and stash your stuff in a small bag — no one likes a huge rucksack-wielding space-hogger.
Many main roads in central London will be closed from 7am, with buses on diversion or terminating early from 6.30am-7.30pm. There are also changes to services on the tube, DLR and some Santander Cycles docking stations will be closed.
Plan your route as much as possible — it's going to be worse than rush hour crush on the Central line. TfL's London Marathon 2016 guide should help — it's got a list of closures and changes. The MBNA Thames Clippers will be running special extra services, so that could be a good option if you're planning to head near the Thames.
THE FINISH: It's a tough call as it's obviously one of the most exciting places, but trust us, it's not worth the chaos.
TOWER BRIDGE: What's not to love, watching the race from a London landmark? Plenty, that's what. Spectators are really keen on this one, so they start queuing from almost the middle of the night. Have 40 more winks and watch from somewhere else with more space and less early-rising required.
MILE MARKERS: 'Meet you at mile 17', 1,000 or so people have already said to their runners before you. It's going to be really hard to spot whoever you're cheering on in a sea of vests with numbers on. Unless they're in fancy dress, in which case — we'll give you this one.
Especially don't go to the area between mile 15 and mile 18, as it's not open this year due to building work.
Places to go
THE START: This is a good call. It's even more exciting than the finish line, with runners lining up, bursting with energy. As there are different start lines (have a look here to see their location), it's not as congested. You won't be able to accompany your runner right to the start line though as it's race entrants only. The mass race starts from 10am this year.
ISLE OF DOGS: The loop south of Canary Wharf tends to be the least crowded of the course.
MUDCHUTE: It's less packed with people here, and it's easily accessible by DLR plus not too far from Canary Wharf. There's also a handy city farm and large park near by, if you're getting cramped by the crowds.
Of course there's always watching it on TV. BBC shows coverage from 8.30am, so you won't miss a thing. It's also on BBC Radio 5 Live and BBC Radio London (check their websites for exact times).
What to do afterwards
Runners with charities normally have somewhere reserved to go and recover, which is perfect. But what do the others do? Every pub, bar, restaurant and coffee shop within a mile of the finish is going to be packed for most of the day. So where do you take your limping runner once they've crossed the finish?
If you haven't reserved anywhere, the best strategy is to choose a pub/bar/whatever right now and book a table. Then — as you're not going to the finish line — head to the place by noon and keep a couple of seats for you and your runner, so they can make their way there without worrying where you are. You'll be following them on the tracking system, so you won't lose each other.
It's also going to be a struggle getting home, so let the crush die down as much as you can before helping your blistered runner towards public transport. Check TfL for the latest cancellations and station closings due to the marathon — they've got loads of useful information here.