You can cross at any time of year, and depending on your boat and your crew, you can (generally) stick to a fixed schedule. I went across once in the classic "don't go" conditions, wind
had been blowing steady from NE at 20+ kts for 3 days, seas were 8-10' N swells and 5-6' NE waves. We were an experienced crew of 4 on a 51' Dufour
(triple reefed), and while it wasn't fun (by any definition), there was never any question of whether we would make it or not. On the other hand, I wouldn't do that on my 36' with ANY crew, and generally the crew I have one board is inexperienced. So whether or not you can do it really depends on your boat and your crew (hurricane conditions notwithstanding...).
If you don't have a big seaworthy
boat and experinced crew, your odds of getting a larger window of moderate conditions are best in the summer, but it can happen in January too. Just have Plan B in mind, especially if you have only 4 days. pete