Parents are often baffled as to why schools in England close for in-service training days, or 'Inset' days, Shadow Education Secretary Tristram Hunt says.
Speaking at a teachers' conference, Mr Hunt said schools had a duty to explain to parents how these days improved children's education.
He said schools should inform parents what skills teachers are developing.
Inset days were introduced in 1988 by the then Conservative Education Secretary, Kenneth Baker.
The training days, which acquired the nickname 'Baker days', were brought in as part of a raft of changes, including the introduction of a national curriculum.
Addressing the Association of Teachers and Lecturers annual conference in Manchester, Mr Hunt said: "If we want to build a progressive case for professional development... then we need to take parents with us.
"And one of the elements of the school calendar that really baffles them is Inset days.
"In my experience, schools inform parents about all sorts of things - from dinner money demands to uniform requirements to recycling rates - but one of the most important elements that boost their children's learning is never explained.
"When it comes to continued professional development (CPD), parents are dealing with an information black-hole.
"So I am calling on school leaders to explain to parents just what is happening in these Inset days: is it focused on numeracy, or behaviour management, or special educational needs provision?
"If we want parents to get behind CPD, if we want to build momentum for CPD - and we do - then let's start informing them what it is all about."
The idea was welcomed by the parenting website Netmums.
'Awkward' for parents
Founder Siobhan Freegard, said: "Inset days can be awkward for parents, especially when they have children at different schools and have to take extra time off.
"Many inset days come joined to holiday time or long weekends, prompting suspicions from parents as to what they are really for.
"In other jobs training is completed as part of your normal hours, so it would be hugely helpful for schools to explain what each inset day is needed for, and get parents to understand and accept why they are vital to the school's success."
The Teacher Development Trust which works for better professional development for teachers said Mr Hunt's comments reflected their own research.
A recent survey of 1,000 teachers by the charity found only 24% thought parents were aware of the professional development staff undertook in their schools.
Chief Executive David Weston said: "I completely agree with Tristram Hunt that parents should care deeply about how effectively their children's schools are effectively developing their staff rather than simply relying on too many one-off courses and generic and superficial tips for teachers.
"Children deserve teachers who are properly supported, and teachers deserve the collaborative professional development that research has shown will really help children in their classes."