On the 10th of next month, the two of them will celebrate their 20th wedding anniversary.
Literal
Next-month 10th [on-に] two-people [topic-は] marriage 20-anniversary [object-を] greet.
迎える ('to welcome / receive / greet') takes on a metaphorical sense here meaning 'to reach [a milestone].' Used for anniversaries, new years, and other arriving time events. 結婚２０周年 is a fixed template for wedding anniversaries. 二人 ('the two [of them]') is read ふたり, a native number.