diff --git a/public/images/20261024_banner_open_day.webp b/public/images/20261024_banner_open_day.webp
new file mode 100644
index 0000000..4f82b3a
Binary files /dev/null and b/public/images/20261024_banner_open_day.webp differ
diff --git a/src/components/molecules/EventDialog/EventDialog.test.tsx b/src/components/molecules/EventDialog/EventDialog.test.tsx
new file mode 100644
index 0000000..0460136
--- /dev/null
+++ b/src/components/molecules/EventDialog/EventDialog.test.tsx
@@ -0,0 +1,61 @@
+import { act, fireEvent, render, screen } from '@testing-library/react'
+
+import EventDialog from './index.js'
+
+describe('EventDialog', () => {
+  beforeEach(() => {
+    vi.useFakeTimers()
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
+  it('appears after a delay', () => {
+    vi.setSystemTime(new Date(2026, 9, 19))
+    render(<EventDialog />)
+
+    expect(screen.queryByRole('img')).not.toBeInTheDocument()
+
+    act(() => {
+      vi.advanceTimersByTime(3000)
+    })
+
+    expect(screen.getByRole('img', { name: /open day/i })).toBeInTheDocument()
+  })
+
+  it('can be dismissed', () => {
+    vi.setSystemTime(new Date(2026, 9, 19))
+    render(<EventDialog />)
+    act(() => {
+      vi.advanceTimersByTime(3000)
+    })
+
+    fireEvent.click(screen.getByRole('button', { name: /close/i }))
+    act(() => {
+      vi.runAllTimers()
+    })
+
+    expect(screen.queryByRole('img')).not.toBeInTheDocument()
+  })
+
+  it('still appears on October 25th', () => {
+    vi.setSystemTime(new Date(2026, 9, 25, 23, 59))
+    render(<EventDialog />)
+    act(() => {
+      vi.advanceTimersByTime(3000)
+    })
+
+    expect(screen.getByRole('img', { name: /open day/i })).toBeInTheDocument()
+  })
+
+  it('does not appear after October 25th', () => {
+    vi.setSystemTime(new Date(2026, 9, 26))
+    render(<EventDialog />)
+    act(() => {
+      vi.advanceTimersByTime(10000)
+    })
+
+    expect(screen.queryByRole('img')).not.toBeInTheDocument()
+  })
+})
diff --git a/src/components/molecules/EventDialog/index.tsx b/src/components/molecules/EventDialog/index.tsx
new file mode 100644
index 0000000..67d668a
--- /dev/null
+++ b/src/components/molecules/EventDialog/index.tsx
@@ -0,0 +1,43 @@
+import { useEffect, useState } from 'react'
+import { Box, Dialog, IconButton } from '@mui/material'
+import CloseIcon from '@mui/icons-material/Close'
+
+const SHOW_DELAY_MS = 3000
+// Local midnight after Oct 25th, so the event is promoted through the whole day.
+const EXPIRES_AT = new Date(2026, 9, 26)
+
+const EventDialog = () => {
+  const [open, setOpen] = useState(false)
+
+  useEffect(() => {
+    if (Date.now() >= EXPIRES_AT.getTime()) return
+    const timer = setTimeout(() => setOpen(true), SHOW_DELAY_MS)
+    return () => clearTimeout(timer)
+  }, [])
+
+  return (
+    <Dialog open={open} onClose={() => setOpen(false)} maxWidth={false} aria-label="Open Day event">
+      <IconButton
+        aria-label="Close"
+        onClick={() => setOpen(false)}
+        sx={{ position: 'absolute', top: 4, right: 4, bgcolor: 'background.paper', '&:hover': { bgcolor: 'background.paper' } }}>
+        <CloseIcon />
+      </IconButton>
+      <Box
+        component="img"
+        src="/images/20261024_banner_open_day.webp"
+        alt="Open Day, October 24th"
+        sx={{
+          display: 'block',
+          width: 'auto',
+          height: 'auto',
+          maxWidth: 'calc(100vw - 64px)',
+          maxHeight: 'calc(100vh - 64px)',
+          objectFit: 'contain',
+        }}
+      />
+    </Dialog>
+  )
+}
+
+export default EventDialog
diff --git a/src/components/templates/RouteTracker/index.tsx b/src/components/templates/RouteTracker/index.tsx
index 49e55a0..4504aac 100644
--- a/src/components/templates/RouteTracker/index.tsx
+++ b/src/components/templates/RouteTracker/index.tsx
@@ -1,6 +1,8 @@
 import { useEffect } from 'react'
 import { Outlet, useLocation } from 'react-router-dom'
 
+import EventDialog from 'components/molecules/EventDialog/index.js'
+
 /**
  * Route tracking component that wraps all routes and includes analytics page tracking
  */
@@ -11,7 +13,12 @@ const RouteTracker = () => {
     window.scrollTo(0, 0)
   }, [location])
 
-  return <Outlet />
+  return (
+    <>
+      <Outlet />
+      <EventDialog />
+    </>
+  )
 }
 
 export default RouteTracker