I suppose your day-job boss would like to be top on the priority. E.g. you have an all important multi-million dollar deal to discuss (which pays you salary of $200 per day) and you have a wedding photoshoot (paying one-time $1000). If you take leave, I am sure your boss will be REALLY upset.
While most work contracts does not specify whether part-time work is ok or not, most professsional jobs will say that you are to hold 'no other employment without the knowledge and agreement of the company'. Let you boss know that your hobby pays, and most bosses will be supportive of the spirit.
If there is clear conflict of interest, or doing a job in the same field/business/profession, then there might be injunctions. Put yourself in your employers' shoes and consider how you'll like your staff to behave.
In my case, a friend working in a government body was getting married, but I turned down shooting for him, because I have so many dealings with his team, his bosses and his other colleagues that it would be really weird to see me shooting his wedding, especially for a fee.