On Earth, we experience seasons because of our planet's axial tilt.
It is a common misconception that the seasons are instead caused by our planet's distance from the sun changing as it orbits. The Earth's distance from the sun does change throughout the year, but the change is far too small to have any effect on the planet's temperature. In fact, in the northern hemisphere, the Earth is actually furthest from the sun during summer.
Imagine an earthlike, inhabited planet with zero axial tilt (ignoring wobble) and a much higher orbital eccentricity than Earth (exact values aren't important at this point but feel free to calculate them for extra kudos). For this planet, "summer" is the time spent closest to the sun and "winter" is the time spent furthest from the sun.
Is this setup a feasible explanation for the seasons of a fictional planet?
If so, how would the planet's seasonal cycle differ from Earth's (all other things being equal)?
Would there be any notion of an equinox?