The Valley floor is a pretty level 4,500 ft. The tops of the points, domes, crests and ridges average 7,200 to 8,500 ft. You won't have any time to go to those altitudes, and the roads getting to the valley floor are higher in altitude then drop into the valley.
There are several Good Sam parks along Hwy 99. Then it's about a 1 1/2 hour drive from 99 to Yosemite Valley. The roads are kept very clear into Yosemite Valley. Since you are only planning a day trip I highly recommend camping off hwy 99 and driving in to Yosemite Valley rather than going real slow dragging the Monty closer since you are looking mostly at a side trip.
I like that Carol reports the weather is nice, but be prepared like it will get cold since weather can change VERY FAST. So keep the rain jackets and warm sweaters and heavy jackets close by; like in your vehicle when you drive there.
NOTE: The day you spend there and you'll wish you spent four or five. Consider this as a worthwhile exploratory trip. As you'll learn when you get there, one of the reasons for its popularity is that it is one National Park where accessibility to what would normally be inaccessible to only the fittest is prevalent. Unfortunately, this time of year, many of these side roads are closed from snow and only the strong-hearted will continue. But from the Valley's floor, you'll be able to see A LOT (just look up) and do a lot in a day.
I took this picture late in the day in late May, 2010. There will probably be more snow during this time of year.
This one, too.
