In the Roman Catholic tradition, when does lent end?

Please provide concrete proof from the Vatican or another reliable source.

When Easter begins!

Easter.

It's traditionally celebrating the 40 days leading up the Easter Festival.

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Lent ends on the vigil mass on Holy Saturday

It doesn't need proof. It's common knowledge that it's Easter.

In Christian countries anyway. We're all taught it at school.

Easter, but it sounds like you have different ideas. Do tell.

It ends at evening Mass on Holy Thursday (the Mass of the Lord's Supper). People get a bit confused partly because the 40 days doesn't include Sundays, and partly because some say it used to continue right up to the Easter Vigil. Certainly, given that Good Friday is a day of fasting, and that many people maintain some sort of fast on Holy Saturday before the Vigil, it is a reasonable assumption to think these are part of Lent (if your primary association of Lent is with fasting). But, as my source describes, Lent ends at Mass on Maundy Thursday.

This is because the Triduum (Maundy Thursday/Good Friday/Holy Saturday Vigil) is a separate tiny season all by itself.

The Catechism actually doesn't seem to specify, nor does the Code of Canon Law. You have to look up The Liturgical Year.

Zog is correct. It ends on Holy Thursday.