Find date of upgrade from Mojave to Catalina

For me, the most direct route to get software update information is using Terminal and the softwareupdate command, e.g.:

softwareupdate --history

To filter for macOS Catalina, use e.g.:

softwareupdate --history | grep Catalina

As shown in the output from my system:

$ softwareupdate --history | grep Catalina  
macOS Catalina                                     10.15      10/08/2019, 07:24:17  
$

You can also use softwareupdate -h to output the internal help. or use man softwareupdate for its manual page.

These are the steps to follow:

1. Go to applications
2. Click utilities
3. Go to System Information
4. On the left scroll down to software
5. Click system installations and sort by software name
6. Look for the update and when it was installed, an example as follows: