Do a little research. Track down the magazine's editorial calendar. Not only does that provide valuable information on upcoming topics, it can also lay out the timing. Some publications work a year ahead, some work six months ahead and others can work just three or four weeks ahead.

Bad timing is a big reason some great queries are rejected. Maybe they just covered the subject, or it would have fit well in the issue they just put to bed. Do your leg work and you can figured out the optimum time to pitch ideas to specific publications.