What's happening
The Sun's ingress into Leo happens when its ecliptic longitude crosses from 30° Cancer into 0° Leo, roughly a month after the summer solstice. Leo is the fifth sign of the tropical zodiac, and the Sun spends about thirty-one days there. In the northern hemisphere this is the heart of summer.
Classical astrology names Leo as the Sun's domicile — the sign it rules. Among all the Sun's yearly transits, this is the one where the tradition treats it as most fully itself.
The tradition
Leo is a fixed fire sign. The tradition associates it with creative self-expression, performance, sovereignty, generosity, play, romance, and the gold at the centre of a chart. Where Cancer protected the interior, Leo steps onto the balcony.
A planet in its domicile is said to carry its themes with unmuddied clarity. The Sun in Leo is the Sun as the Sun: vitality, radiance, visibility, and the honest question of what deserves to be at the centre of a life. The tradition treats this month as an annual check-in with one's own sovereignty — not ego in the dismissive sense, but the dignity of a self that is willing to show up and be seen.
How to work with it
Make something public. Finish the project you have been polishing in private and let it out of the room. Take the credit you have been deflecting. Throw a dinner. Wear the thing. Leo rewards visibility, and specifically the kind that comes from generosity — the bright presence that makes other people feel more vivid in return.
The shadow is vanity, or its flipside — the performance of modesty that is really a fear of being seen. Leo's real test is whether you can receive attention without tightening around it, and whether you can give it as easily as you expect it.
The simple rule
Ask what in you wants to be visible and has not been. The Sun in Leo rewards the answer you are slightly embarrassed to say out loud.