From the SEIS help desk:
SEIS typically rolls out updates for the site twice a month. These rollouts usually occur on Thursdays between 7 to 9 pm. If a rollout is going to occur that day, a red banner will appear at the top of SEIS informing users of it.
During a rolllout, the SEIS site does not completely go down. However, there is the risk of data loss if you are working while a rollout is actively updating the SEIS site.