Yes, they can take it as a vacation day. They can take a vacation day if they want. They can do it today, on Monday, or any other time of the year, providing they get the permission of their manager. In each case it is a personal choice. Nobody is being forced to do anything.
A lot of people who work in the public service feel strongly about the country and want to be part of today's rally in Montreal. They are doing it all in accordance with the rules, practices and collective agreements of the federal government.