Surya (Sun) is currently transiting Hasta Nakshatra. Hasta Nakshatra is connected to our Hands. So the practice of Mudras comes under the domain of Hasta Nakshatra. Sunday is a ruled by the Sun as well. So performing Surya Mudra on Sunday helps you Honor the Sun God.
Surya Mudra is one of the most powerful Mudras and it can be used to handle lethargy, tiredness, exhaustion etc. It also gives you the inspiration to achieve your life goals and energises you both physically and mentally.
Ring finger is connected to Sun and Thumb is connected to Mars. When you bring both together it is almost bringing a King (Sun) and his commander in chief (Mars) together. Thus, under the guidance of the king, the commander in chief can perform his duties. And with the protection from the commander in Cheif (Mars) the King (Sun) is able to rule and perform his duties as well.
How to do this Mudra
You bring the ring finger to touch the root of the thumb and press your thumb against the ring finger to perform this mudra.
You can do it with one or both your hands in a seated position.
Combine this with Pranayama or just deep slow breathing to maximise the benefits.
Additionally you may also chant the most profound Gayatri Mantra to invoke Lord Savitur (deity of Hasta Nakshatra) to help you manifest your intentions. The power or Shakti of Hasta Nakshatra is Hasta Sthapaniya Agama Shakti which means to Gain what we are seeking and place it in our own hands.
Since Mars and Sun are fiery planets and represents the Pitta constitution continued practice of this Mudra can help you with digestion and weight loss as well.
But take precaution not to perform this Mudra when you are hungry and if you are dominated by a lot of Pitta energy as that may get aggravated.