There is no question that Easter Day in 1963 fell on Sunday, April 14. My Catholic
New Marian Missal printed in 1952, and my Episcopalian
Book of Common Prayer printed in 1953 both confirm April 14 as the correct date for that year. Plus, there are extant
photos and
video of President JFK and his family celebrating what would be his last Easter on that date.
Easter Day may seem unpredictable, but calculating its date is really no secret. Easter is a full moon holiday, and the Church uses a repeating series of 19 dates to approximate the full moon, the Sunday after which is Easter. In 1963, the full moon date was Monday, April 8—hence Sunday, April 14 was Easter.
Here's how I make sense of the seven-day discrepancy from the info on the Doe Network:
I think Sunday, April 7 is when the
neighbor last saw Jane Clement. Then "a week later [i.e, Easter Week, April 14-20], a worried neighbor [said] she hadn't seen Clement in days." The police began to investigate shortly thereafter. When they questioned the husband, he said he had seen her just the other day—Easter Monday, April 15—when she asked him to watch the children.
Maybe because the police suspected the husband, they list April 7th as date of last contact since this comes from an independent and impartial source. I think the newspapers simply confused the two accounts, and produced the erroneous "Easter Monday, April 8th."
Note that the details about Jane's call to her husband and the Easter dress come only from him, so they can't be independently verified, especially since the dress conveniently disappeared. Maybe there was no Easter dress, and he contrived that detail to add weight to his own story and play down the neighbor's concerns that Jane hadn't been seen in a week, that is, from April 7th.
I think another clue comes from the washed but mildewed clothes which Jane's brother found when he broke into her house. If the neighbor called the brother "one week later," and this refers to Easter Week, April 14-20, then this backs up the neighbor's story because one week is enough time for mildew to form. I doubt a worried brother would've waited a week
after the call to go to the house, so the clothes were left unattended from around April 7.
How young were the children? If they were old enough, surely they'd remember spending a whole week with their dad and not their mom. Then again, if they were young enough and not as aware of time, he could've simply convinced them that he had picked them up on Easter Monday, April 15th when in reality it had been one week prior.
ETA: I found a Reddit thread that describes the children as "babies." I know Reddit isn't the most reliable source, but given that Jane went missing at 22, it makes sense.