When to give notice at a Big 4 during busy season?

I know that I need to leave my job in 3 to 4 months, but I just don’t know when I should let my workplace know. I am getting married and my coworkers and leadership know, but what they don’t know is thay I am moving to my fiancé’s city (across the country). I want to quit sometime in April; my department is on the smaller side and the busy season runs from January to June. Knowing there’s never really a good time unless it’s the summer months, when should I let them know? (Transferring is not an option, as they don’t have an office where I’m moving).