All options are either American or European style, which dictates when the option can be exercised or assigned. (It has nothing to do with geography.) All equity (single-stock) and ETF options are American style. American style options can be exercised or assigned at any point in time on or before expiration. SPY (ETF) options are American style, meaning the option owner may choose to exercise ahead of expiration. For example, this often occurs in advance of the quarterly ex-dividend date.
The ex-dividend date is the first trading day when a dividend-paying stock or ETF’s price drops to reflect its next dividend payment. So, if an ETF pays a $0.25 dividend, the price may decline by that amount prior to trading on the ex-dividend date, barring other market factors.
If an option is in the money going into the ex-dividend date and the dividend exceeds the remaining time value of the option, the call owner likely has economic incentive to exercise their options early. This leaves the seller in the position of having to deliver the shares as well as the dividend.