Pre-Season waiver claims can now begin. Each team is allowed one pre-season Add/Drop. You can put in multiple claims as long as you use the same player to drop in each claim. Using different players to drop (for multiple claims) will likely result in you getting more than one new player.
There are actually four different claim periods this week,
Monday, Tuesday, Wednesday - results on Thursday @ 1:00 AM ET
Thursday - results on Friday @ 1:00 AM ET
Friday - results on Saturday @ 1:00 AM ET
Saturday - results on Sunday @ 1:00 AM ET

There will no claims on Sunday, April 4th as it is Opening Day. Regular season Add/Drops, Add/DLs, and Add/MLs can be done on Sundays, beginning April 11th. Claims put in on Sunday will have results on Monday @ 1:00 AM ET.