On the first Thursday of every month there is an update released by Facepunch which has the side effect of a "forced wipe". This essentially means that every server is forced to wipe their map save and start fresh. Blueprints are not included in the forced wipe and the server owner can choose whether or not they'd like to keep blueprints on their servers. We choose to wipe blueprints on all of our servers at the time of update (forced wipe).
While Facepunch tries to release the update at about 2PM EST (19:00 GMT), there is no set time and this can be off by many hours. We will not know when the update is coming out until a few minutes before it is released to Steam.