Aim for the week before most high schools and colleges are on Spring Break. I went this past March during that week (I believe it was the second week of March) and while there are still crowds they are not nearly what they are capable of. That is also when the summer hours are rolled out so the park opens earlier and closes later. They bring the fireworks and Fantasmic in on a regular basis instead of just weekends, as well. Best time of the year to be at Disneyland, imo.
Just make sure you bring a sweatshirt. I just about froze the first day I was there. It didn't get above 70-72 degrees the whole week.
Depending on when you go and how many are in your group (kids? adults only?), 3-4 days at least would be ideal. That way you would have plenty of time to see everything, redo things you really like, go back to the hotel for swimming (weather allowing) and rest time, and see the shows (WOC, F!, fireworks). Remember that if you go during the off season hours, those shows are Fri. through Sun. only.
Interesting... 4 out of the 5 times I've made a trip from Canada to the US and spent a few days in DL have been in March. All the days I went landed on the last week of March.
I can tell you from personal experience that it will be busy, but manageable in my opinion. Of course, now that Cars Land is open March may be a whole different story BUT, let me tell you this... LAST March, I went for three days in the last week of March and, surprise surprise, the park was actually fairly quiet. I don't remember a line going above 50 minutes and to put it into even more perspective, I was still getting fastpasses for Indiana Jones at around 6 PM! Usually they're gone by around noon! I don't even remember Thunder Mountain running out at all!
That being said, Cars Land could probably change March quite a bit. My advice to you is, even if it rains, still go to the park. March 28, poured rain the night before and early in the morning. Aorund 7 AM, when I left for the park, the rain stopped and by the time I was on the tram the sun was breaking through, and hardly anyone in the park until about 11 AM.
A big thing to remember is plan for 2013 and not base it on other years. Because Spring Breaks generally happen due to Easter. Many schools slide thier breaks forward or back to coincide with the holiday. This year it's a little early.
For March crowds:
1st week is good
2nd week ok
3week, almost ok not quite bad
4th week very bad
So you can pick based on crowds, but I would consider what Dr. Fink and PacalGal mentioned which may be important. Entertainment will pick up and hours get longer likely in that third week. Also rehabs should finish for that week so more attractions in operation.
If you are only going for a couple days midweek will have the lower crowds, but you can mix them early in the month to get long hours and entertainment by going Thursday/Friday or Sunday/Monday.
Check the refurb schedule as often as you can. Thunder Mountain and Alice are going for rehab downtimes soon and will be lengthy ones. Other refurbs will be announced as their scheduled downtimes are announced.
As of right now, the only rehabs announced for March are for the Mark Twain, Big Thunder Mountain and the Fantasyland Theater. The Mark Twain is scheduled to reopen on March 29th the other beyond your visit period. Of course the Princess Fantasy Faire has closed and is moving into the old Carnation Plaza but won't reopen as Castle Fantasy Faire until 2014.
If you are worried about rehabs you could make cancelable reservations if you want to lock in prices now and then evaluate if you should change them when more is known. But like I said most will wrap up for later in March when more crowds arrive.
I wouldn't let refurbs ruin your trip. This year I had the Matterhorn Bobsleds closed my first trip (not to mention Cars Land and Buena Vista Street weren't done yet) and Indiana Jones, Storybook Canal Boats, and Casey Jr. closed on my second trip. The bottom line is you still manage to have fun with what you have available.
If you are staying in one of the resort Hotels, take advantage of the Early Morning entry (think that is what it is called....) I never get up early enough to go in at 7 a.m., but made the effort for DCA and it was VERY worth it. We got to ride Cars three times in the 45 minutes before the regular crowd came in. We were having breakfast at Flo's when the castmembers were walking bckwards with the rope holding the people back from sprinting to the ride! We spent the whole day in DCA, which we usually don't do - we usually park hop - and had a great day. Spent the other 2 days of our trip in Disneyland.
Have a great time!
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