Can a lay person observe the ten precepts on uposatha days? I ask this because I want to observe the eight precepts on uposatha days and I don’t need to use money on those days anyway so I might as well observe the ten precepts instead of eight. So is it ok for me to observe the ten precepts or should the ten precepts be only observed by novices and nun?
It's just like the case where monks need not do more than what is required by the Vinaya. But if they want to, they can optionally choose to observe the stricter Dhutanga rules.
In any case, it is useful to remember that the Buddha taught the Middle Way between excessive indulgence and excessive austerity.
The analogy from the Sona Sutta is applicable here to illustrate that energy and effort applied to the practice should not be too much or too little:
“What do you think, Soṇa? When you were still a layman, weren’t you a good harp player?”
“When your harp’s strings were tuned too tight, was it resonant and playable?”
“When your harp’s strings were tuned too slack, was it resonant and playable?”
“But when your harp’s strings were tuned neither too tight nor too slack, but fixed at an even tension, was it resonant and playable?”
“In the same way, Soṇa, when energy is too forceful it leads to restlessness. When energy is too slack it leads to laziness. So, Soṇa, you should apply yourself to energy and serenity, find a balance of the faculties, and learn the pattern of this situation.”