Lancet Oncology is a monthly publication, but the online edition is typically published prior.
The Lancet is a weekly publication with the online edition being available 'a few days prior'. I just spent a few minutes browsing their website, but I wasn't able to find out which day of the week.
Articles in The Lancet journals are published online on a daily basis, ahead of subsequent print publication. Our standard embargo time is 6.30pm [US ET] or 23.30 [GMT], which corresponds to the time the article is published online. Occasionally, embargo times may vary.https://info.thelancet.com/press-room