One of the beautiful things about cruise vacations is that you can find cruises departing for exciting destinations year around. There really is no "best time of year to cruise" because there are always ships departing.
If you want to take a trip next week, you can find a last minute cruise to sign up for. If you want to plan a trip for next winter, you can also find a cruise to book.
It really just depends on the destination you have in mind. Naturally some places have limited cruise seasons (most people want good weather for their trips).
For example, if you want to cruise to Alaska, you'll need to book a trip in the summer, between May and September. If you've always wanted to see the nature and wildlife of Antarctica, you'll have to visit in January or February (which is summer down there). What if you just want to head some place tropical? You can take a cruise to the Caribbean any time of year.
For more on when you can cruise where, check out my earlier post that lists Cruising Seasons for Popular Cruise Destinations.