Nesting if statements to echo only one string based on what tag was filtered?

is_tag() returns whether the query is for an existing tag archive page. This isn’t what you want to use.

Instead, use get_query_var().

get_query_var( 'tag' ) will look at the tag query variable in the URL. For instance, if your URL is

https://example.com/?tag=this&other=that

then using:

• get_query_var( 'tag' ) will return this
• get_query_var( 'other' ) with return that

Instead of using a lot of ifs, we can use the PHP switch construct.

//* Use switch instead of a long series of ifs
$tag = get_query_var( 'tag' );
switch( $tag ) {
  case 'northern-california':
    $tag = 'Northern California';
    break;
  case 'seminar':
    $tag = 'Seminar';
    break;
  //* Etc.
  default:
    $tag = 'All';
    break;
}
//* Do something useful with $tag

But to use get_query_var() for a custom query variable, we need to add the query var to the array of variable names that WordPress will retrieve using the function. This can go in your theme functions.php or in a simple plugin, as long as it’s loaded before you try to get_query_var().

The string name used in the get_query_var() function must be exactly the same as the name used in the filter. If you want to use eventTag as your query var, then you need to add that to the query_vars array and use that as the query var in the URL:

https://example.com/?eventTag=california

This would go in your functions.php file

//* Allow custom query var 'tag'
add_filter( 'query_vars', function( $vars ) {
  //* Use whatever custom query var you want, but it needs to be exactly the same
  $vars[] = 'tag';
  $vars[] = 'eventTag';
  return $vars;
}, 10, 1 );

If you don’t like using closures:

add_filter( 'query_vars', 'wpse_261930_query_vars', 10, 1 );
function wpse_261930_query_vars( $vars ) {
  $vars[] = 'tag';
  return $vars;
}

It’s unclear how you would structure your URL for two different tags.