Gmail – Why Does My Gmail Icon Have Balloons?

Gmail celebrates 15-year anniversary. This can be briefly spotted upon login to Gmail for few seconds on loading screen (if you reload your Gmail with F5 key)

Back in 2004, email looked a lot different than it does today. Inboxes were overtaken by spam, and there was no easy way to search your inbox or file messages away. Plus, you had to constantly delete emails to stay under the storage limit. We built Gmail to address these problems, and it’s grown into a product that 1.5 billion users rely on to get things done every day. Today, on Gmail’s 15 birthday, we’re taking a look back and sharing where we’re headed next.

On April 1, 2004 we launched Gmail (despite the timing, not a joke). It had the power of Google Search built right in and grouped your messages into conversation threads, making it easier to find and reply to them. You could also store 1GB of data for free - nearly 100 times what was available at the time. No wonder the world thought it was a prank.

https://blog.google/products/gmail/hitting-send-on-the-next-15-years-of-gmail/

Also, it's worth to mention that Gmail got updated with two more futures:

• schedule delivery of messages
• respond to a comment thread in Google Docs/Sheets