In the first game of the day, Corpus Christi take on Glantaf in the Under 14 final before YG Gwyr clash with Cowbridge in the climax to the Under 16 competition.
"Inter-school rugby is a vital component in the development of the game in Wales and we are delighted to be able to host the Welsh schools U14 and U16 Cup finals at the Millennium Stadium," said WRU Rugby Participation Manager Adrian Evans.
"They represent the pinnacle of interschool rugby fixtures and all finalists are to be congratulated on their achievement of reaching this stage.
"The hard work of the teachers in the running of these teams is a shining example of the contribution they make to the Welsh game."
Vale Schools take on Newport in the DC Thomas final before Carmarthen meet Gorseinon to round off the day.
"It's a culmination of a year's competition throughout Wales," said Lyn Howells from the Welsh Schools Group.
"It started before Christmas and it is very much a national completion.
"There were two fantastic semi-finals in the Under 16 age group and it augers extremely well for the future.
"If those games are anything to go by then there will be some great rugby on show tomorrow."
Entry to the Millennium Stadium is free on Saturday with the first game kicking off at 10.00am.
10.00am: U14s Corpus Christi vs Glantaf
11.30am: U16s YG Gwyr vs Cowbridge
13.10pm: DC Thomas U11s Vale Schools vs Newport
14.30pm: Youth U19s Carmarthen vs Gorseinon