One of the key dates in the English football calendar is almost upon us. Third-round day of the FA Cup takes place on Saturday and as well as a few interesting looking all-Premier League ties there is also plenty of potential for a classic cup upset.

Although no top-flight side are playing a genuinely low-league opponent, there will be one or two teams from England's top three leagues who might well be concerned about making the headlines for the wrong reasons. Here, Sports Mole picks out three clashes that could see the smaller side advance to the fourth round.

1. Charlton Athletic vs. Oxford United

The key ingredient for a cup upset is a team from a high division struggling in their league coming up against one from a division or two below who are performing. This match has that. Although Charlton have made themselves tough to beat lately with just one defeat in seven they are only three points above the drop zone and could be ripe for the picking.

What's more is that there is talk of a potential takeover at the Addicks, which manager Chris Powell himself said could increase the uncertainty over his job situation. If Oxford want to take advantage of this, they will need to recover from their own patch of bad form. Just one win in six has seen them slip from the automatic promotion places to the playoff spots but if this is anything like a real cup tie league form goes out of the window.

What will encourage Charlton as they look to avoid what would be a shock defeat, is that the two sides have already clashed this season, despite being separated by two divisions. The League Two side travelled to The Valley right at the start of the season and ended up on the wrong end of a 4-0 hiding. Much has happened since then which, added to the magic of the FA Cup should see at the very least a tight affair on Saturday.

Sports Mole says: Strong chance of an upset.

2. Kidderminster Harriers vs. Peterborough United

Peterborough United have responded from their relegation from the Championship well. Although they have not quite taken League One by the scruff of the neck in the same way that Wolverhampton Wanderers have, they are still well-placed for a promotion push. That is despite defeat in all three of their festive period games in the league. What they do not need right now would be a humiliating defeat to non-league opposition to further dent their morale.

Enter Kidderminster Harriers. They would want to be coming into this game on the back of a good run of form, however they too are struggling. A miserable run of five defeats from six has seen them slip outside the playoff spots and having lost 6-0 to Luton Town in their last game it could be a concern for them that they now face strong league opposition.

Nevertheless, there is an interesting twist that could go in their favour should omens mean anything in the FA Cup. Barry Fry, now director of football at Posh, suffered defeat to the Harriers as manager of Birmingham City in 1994. He will not want to suffer the same fate again.

Sports Mole says: Upset looks very unlikely.

3. Aston Villa vs. Sheffield United

The side from the Premier League that looks most at threat against opponents two or more divisions below them looks to be Aston Villa. In the league, they managed their first win in six away to Sunderland on New Year's Day to halt their slide down the table, but they are not nearly playing to their full potential this season. In particular, they have struggled against teams that keep things compact in defence.

Against Sheffield United, this is exactly what they should expect. The League One side moved themselves clear of immediate relegation danger with an unbeaten run of seven games before defeat to Walsall on Wednesday. However, with three clean sheets in that run they certainly will not go all out to attack their more illustrious opponents.

Villa have been much better away from home this season, when they have been able to sit back and attack on the break, but on Saturday they will need to do the pressing in order to get the win. An early goal would go a long way for them, but the longer this one goes on without a goal the better the Blades' chances get of causing what would likely be the upset of the round.

Sports Mole says: Villa should have enough.