Grandpa is the first character players are introduced to. He writes a letter gifting the farmhouse, which prompts the player to move to Stardew Valley.

The player can find a note on Grandpa's Shrine, at the far Northwest corner of The Farm, which states that Grandpa will return at the start of the third year (1st of Spring, Year 3) to evaluate the player's success.

Grandpa's evaluation is directly related to the number of candles that light up around Grandpa's Shrine (there are 4 candles) at the beginning of year three. This is determined by a point system based on various milestones that were reached in the first two years. There are a total of 21 available points.
The player can be re-evaluated at any time by placing a Diamond into Grandpa's Shrine (Grandpa will show up on the following day, after a game load or the first day of the following season in some cases). This means that the opportunity to obtain all four candles is extended beyond the initial two year evaluation.

Source - Stardew Valley Wiki