You've probably seen people sharing 420 and wishing others a Happy 420 day!
If you're a bit baffled by it all don't worry, it's all because April 20 is the unofficial international Weed Day.
The code 420, also seen as 4:20 or 4/20 and pronounced four-twenty, is a code-term for cannabis culture that refers to the consumption of cannabis.
It specifically refers to smoking of it at the time 4:20pm and the day April 20, which is 4/20 in US date form.
It comes from somewhere in San Rafael, California, in the late 70s, as a code the police used for Marijuana Smoking in Progress.
When locals heard the police call, they began to start using the term when referring to Cannabis.
The 'celebration' has now grown, and is marked today with rallies, parties and protests.
People meet to smoke Cannabis and advocate for it to be legalised.
In nine US states as well as Washington DC the recreational use of cannabis is legal for people aged 21 and over.
Twenty-nine other states allow it for medical reasons.
Where did 420 start?
There are plenty of urban legends and theories about where it all began.
The main one is the police code used in the 70s that we've already mentioned.
A specific instance is referenced. In 1971, students at a high school in California apparently organised to meet at 4.20pm to find a plot of land to plant a cannabis plant.
The group is called The Waldos, five friends who met up at a wall.
One of them, Dave Reddix, spoke to Sac Bee said they went for a hunt for a hidden weed garden planted in a forest. The journey began at 4.20pm. The code stuck.
While most agree this is where the first use of the term came from, others have challenged the story.
In 2017 the Oxford Dictionary settled the matter bu interviewing The Waldos and including the term in the dictionary.
When is the code used?
People often use it as a code phrase, to meet up, or on dating sites to show they're ok with cannabis - "420 friendly". Businesses use it in their names to as an indicator.
How do people celebrate?
There are rallies held worldwide, but the biggest UK event is in Hyde Park.
Thousands turn up to light up, despite police warning against turning up to the meeting.
There's even a Facebook page where people have indicated their interest.
The event kicks off at midday. Police will be present and the usual drug laws will still apply.
What does it do to your health?
According to the NHS cannabis can make you feel "chilled out, relaxed and happy" but it has adverse effects.
You can feel dizzy and sick, tired or suffer from memory loss. Long-term it raises the risk of psychotic illness and exacerbate existing mental health issues.