How to block the dates on Houfy?

Open your calendar at https://www.houfy.com/multicalendar (Multi Calendar in the left menu)

Click on the date and select the dates

Click Blocked

Click on Save to block the dates. The panel that opens shows the Start date and End date you selected, an Available / Blocked pair of radio buttons, and a Rule sets dropdown. Once saved, blocked nights appear struck through and shaded on the calendar.