I use an Amazon Dot to control my smart home but it’s also handy for other tasks like checking your calendar. Today Amazon has added the ability to link your Microsoft hosted calendar into Alexa along with the existing Google connection.
From the Settings section of the Alexa app (or browser) you can select your Microsoft account which could be Hotmail, Outlook.com or Office 365 account. It seems like you can only have one Microsoft account connected at one time but you can have both a Google and Microsoft account setup simultaneously. You can also select which is the default account for new calendar events via Alexa.