edit/ I'm not letting you off this easily. Mono never really worked and never really had Microsoft's support. Where's the ecosystem?
Yes, Mono had issues as I said but it did/does work if you can work around it. Granted the windows licenses are cheaper than the dev effort to fix any potential issues so it's not the best situation.
But .NET Core can be used today and it works fine with the framework rapidly being developed. Several major libraries are also in or have completed the migration and we're just waiting on some other dependencies but we do have smaller apps running today.
Ecosystem for the code is the same (and as vast) as it's ever been, that's the strength of the CLR/framework. Mono/.net on linux does have a community but it's growing quickly now with Core.
"one-day" is today.