Matterhorn goes down at least once a year for rehab. no matter how long the previous years rehab was it will still go down. Sometimes it goes down in may, but usually september. It's too early to say what exactly will be down, but I would guess that you will see 4 or 5 rides in DL, and probably at least 2 in DCA.
Sept to Nov is there biggest rehab time as they have almost 3 full months. Then they have a few weeks between Thanksgiving and xmas. Then they have between 2 to 3 months between jan and easter, but they do try to have a lot open for Presidents day, and with Haunted Mansion and Small World down at the same time, they can't have as many others down. Then there's easter to June. Also Sept to Nov is usually a slower time of year which equals more closures.