Expanding upon input from stedman1...
Calendar Infection
Whilst not a malware infection in the traditional sense, if this exploit is observed on your device, it is highly probable that you were manipulated (via a simple click on a website link) into “subscribing” an additional (unwanted) Calendar to your device - and this unexpected Calendar is exposing unwanted calendar events and sending you unexpected “adverts” or other warnings.
If you see this issue, you’ll need to check for what’s out of place...
iOS/iPadOS13 and earlier: Settings > Passwords and Accounts
iOS/iPadOS14 and later: Settings > Calendar > Accounts
Look for an “account” that shouldn’t be in the list of accounts - as this will likely include the Calendar that contains all the unwanted events. When/if you find the suspect account, tap - then select Delete Account. This should resolve this specific problem in its entirety.
Malware
Other than compromised websites, a high percentage of exploits and threats to which you will inevitably be exposed will ultimately manifest via links, alerts and pop-up messages delivered via links embedded within email, messaging - and subscribed Calendars - these often being designed to scare the unwary into giving away sensitive information, or to fool you into doing something that you shouldn’t.
Due to the system architecture of iOS/iPadOS, unless jailbroken, your iPad is not susceptible to traditional malware infection per-se. However, as with all computer systems, there are still vulnerabilities and exploits to which you remain at risk.
Browser-based attacks can largely be mitigated by installing a good content and Ad-blocking product. Of those available, one of the best and most respected within the Apple App Store - explicitly designed for iPad, iPhone and Mac - is 1Blocker for Safari.
https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024
1Blocker is highly configurable - and crucially does not rely upon an external proxy-service of dubious provenance. All processing takes place on your device - and contrary to expectations, Safari will run faster and more efficiently.
Unwanted content is not simply filtered after download (a technique used by basic/inferior products), but instead undesirable embedded content blocked form download.
When using a good quality Content blocker, a high proportion of otherwise inescapable risk when using your Safari browser, or linking to external sources from email, is effectively mitigated before it even reaches you.
There are additional protections that can enhance protection further, such as using one of the better security-focuses DNS Services in preference to automatic DNS settings. This measure can either be set on a per-device basis in Settings, or can be set-up on your home Router. I recommend using one of the following services, for which IPv4 ad IPv6 server address are included here:
Quad9 (recommended)
9.9.9.9
149.112.112.112
2620:fe::fe
2620:fe::9
OpenDNS
208.67.222.222
208.67.220.220
2620:119:35::35
2620:119:53::53
Cloudflare+APNIC
1.1.1.1
1.0.0.1
2606:4700:4700::1111
2606:4700:4700::1001
Security focused DNS providers intentionally "sink hole" known bad or malicious websites and resources - this providing an additional layer of protection beyond that provided by your device and its Operating System. These DNS services will, when used alongside 1Blocker or other reputable Content Blocker, provide defence in depth.