If you look here
http://www.mouseplanet.com/search.php?type=c&cid=dl you can find refurbs in years past.
In general refurbs are heaviest starting in January and up through Spring Break and sometimes into May as well as Labor Day up thru Thanksgiving.
If they need to a major four month refurb like they did on Splash Mtn in 2007 or POTC in 2006, that will generally be in the first part of the year.
High season at DLR is Spring Break (mid-March to Mid-April), Summer (mid-June to late August) and Holiday (Thanksgiving to New Years). DLR tries to have as few rides down as possible during this time to handle the greater crowds. If they have a four month refurb there is no way to fit it in without overlapping high season somewhere, and so they usually do that from Jan-May and have it be down during Spring Break. If memory serves Splash Mtn was down from mid-Jan to mid-May in 2007 and POTC was down Feb-June in 2006.
In general if you want the maximum number of rides and shows operating go in high season (see "HydroGuy's High Season/Off Season Tradeoff Rule"
www.disboards.com/showthread.php?t=1199852). If you want to avoid high season but have the maximum chance of rides being up, then April or May (with May being better) is probably your best bet.
Last time major rides were refurbed? I cannot remember all exactly so others please correct me if I am wrong. I mentioned Splash and POTC already. Indy has been a long time and there were rumors it was going down in 2008 but it never did. Screamin was down for a long time (4 months ?) in 2005 after the accident and had some refurb done at that time. Matterhorn was down in Fall 2007. Can't remember the last time BTMRR was down for a long refurb. Space Mtn has not been down since it was re-opened in 2005.