TL;DR
Most days, in Stardew Valley village Robin is at the Carpenter’s Shop in the Mountain area from 9:00 AM to 5:00 PM. The shop is closed on Tuesday, and it closes early on Friday at 4:00 PM. If it’s raining on Tuesday, the shop is open like a normal day. If you ordered construction, she may be working on your farm instead of running the counter.
Robin’s main location
In Stardew Valley Robin lives and works at the Carpenter’s Shop (24 Mountain Road), north of Pelican Town. This is where you go for farm buildings, house upgrades, and buying wood or stone.

From the farm, the route is simple: head into town, go north past the Community Center, then enter the Mountain area. Her home and shop are on the right.

When the Carpenter’s Shop is open
On a normal day, you can use the counter from 9:00 AM to 5:00 PM.
Two timing rules catch most players:
There’s one extra exception worth knowing: if it’s raining on Tuesday, the exercise class is canceled and the shop runs its normal hours.
Where Robin goes when she isn’t at the counter
Tuesday
On Tuesdays, she leaves for her weekly exercise class during the day, so the shop doesn’t operate like normal. If you absolutely need to interact with the counter on Tuesday, there’s a tiny window when she walks past it in the morning and again when she returns at night, but for most players it’s easier to plan around Wednesday.
Friday after 4:00 PM

On Fridays, she leaves early and spends the evening at the Stardrop Saloon. If you need a building started or materials purchased, go before 4:00 PM.
Rainy days
Rain makes her easier to track. She tends to stay inside her home and shop rather than wandering outside after work. The big one to remember is rainy Tuesday, because it flips the usual rule and the shop is open.
Festival days
Most festivals disrupt normal schedules. If the town is in festival mode, assume the shop won’t run as usual and handle building plans the day before.
When she’s building something on your farm
If you ordered a farm building or a house upgrade, she works on your farm during the build days. During that time, you can’t use her shop services, so don’t schedule construction on a day you also need to buy materials or start another project.
One more schedule quirk

Robin has a clinic visit on Summer 18, which can also throw off a routine visit. If everything else looks right but the shop feels unavailable, check the calendar.
The fastest way to find Robin today
If you want to stop wasting trips, run this quick check:
If you ordered construction recently, check your farm first.
If it’s Tuesday, assume she’s away unless it’s raining.
If it’s Friday after 4:00 PM, check the saloon.
If there’s a festival, she’s at the event.
Otherwise, visit the Carpenter’s Shop between 9:00 AM and 5:00 PM.