My last slow period was 2009.  As far as seasonal swings, I used to see things slow a bit in summer but most of my jobs now are multi-year projects so while there might be breaks between phases the time of year doesn't really have much impact.

For the last 3-4 years I've been booked out solid a minimum of 4-6 weeks and often 3-4 months.  Right now I'm jam packed through June.