You should only sign up for about one month in advance. The phase of the moon will also be indicated on the calendar. The best time to visually observe is 1 week before and after new moon, and moon photos are best 4 days before and after 1st quarter. We'll discuss this more in lab. If you sign up for a night and it is mostly cloudless that night, you must take advantage of it and observe.
Clear nights are far too infrequent to waste.
If the night you signed up for is mostly cloudy, then you do not have to show up in the Observatory that night. In this way you will know which Lab Assistant will be working with you that night in case you need to contact them about meeting times, etc.