While strictly speaking, app recommendations are off-topic here – let's tread it as "how can I do that". And as this is my second question today, it's time for the app which answers every second question here: tasker, optionally with the AutoVoice addon for more convenience and flexibility.
The Tasker UserGuide explains:
Insert an entry into the specified calendar.
(More details there)
With AutoVoice, you can have a widget/shortcut on your homescreen to activate it (or have Tasker do so via some other event, e.g. "shaking the device up-down twice"). Then speak your command, and Tasker would execute the Task assigned to what it recognizes. There are tutorials around also explaining how to make a "loop" for the "Sorry, I didn't understand – please repeat!" case ;)
Drawbacks: Both apps come with a fee if you want to really use them. Tasker has a 7-day-trial at their homepage, AutoVoice has a limited "free version" – so you at least have a chance to check first if it meets your requirements. Will take a few days getting used to Tasker – but once you did, you will wonder how you could do without ☺